FOUS11 KWBC 041953
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
253 PM EST Wed Dec 4 2024

Valid 12Z Wed Dec 04 2024 - 12Z Sat Dec 07 2024

...Michigan...
Days 1-2...

The Arctic front associated with a deepening area of low pressure
tracking to the north of the Great Lakes today is moving through
the Upper Midwest this morning and will soon be the catalyst for
yet another lake-effect snow (LES) event across much of the
Michigan U.P. and for both northern and western communities of=20
Michigan's Mitten. At the onset today, a series of potent snow=20
showers along the Arctic front will race south and east as NWrly=20
winds begin to kick-start the lake-effect snow machine from the=20
Porcupine Mountains to the Hurons. These squalls will work their
way through the southern part of the state this afternoon and
evening with the favorable combination of strong frontogenetical=20
support at low levels and a robust 500mb jet streak (exceeding=20
observed 500mb winds for this time of year in the CFSR database 00Z
this evening) fostering a favorable setup for sustaining potent
snow squalls through this evening. Meanwhile, cold air advection=20
(CAA) over the warm waters of Lakes Superior and Michigan will=20
cause surface-850mb delta Ts to range between 34-40F. These steep=20
lapse rates combined with well saturated low-level profiles will=20
support multi-banded lake-effect streamers that produce 1-2"/hr=20
snowfall rates through tonight and into Thursday. Some hi-res CAMs=20
suggest the potential for a dominant single band streamer that=20
could produce heavy snow in southwest Michigan and into the=20
Michiana area bordering IN/MI.=20

LES should wind down Thursday night and taper off across much of
Michigan by Friday afternoon. WPC PWPF depicts high chances (>70%)
for snowfall totals >8" for much of the Michigan U.P. and in the
northwest part of the Mitten. Some localized areas could approach
18" in the hardest hit spots. There are low-to-moderate chances
(30-50%) for snowfall >8" where the single lake-effect band sets up
in southwest Michigan. The WSSI shows primarily Moderate Impacts
for most of these aforementioned regions, although some localized
Major Impacts (considerable impacts such as dangerous to impossible
driving conditions) are possible in parts of the Michigan U.P. and
northwest portion on Michigan's Mitten.


...Eastern Great Lakes and Northeast...=20
Days 1-3...=20

The types of snowfall from the eastern Great Lakes through the
Northeast Thursday are WAA/Synoptic scale, lake-effect, snow=20
squalls, and upslope mountain snow. The LES and mountain snow are
forecast to persist into Saturday.

1. WAA/synoptic scale: This pattern is driven by a powerful mid-
upper level low over MI this evening before reopening into a=20 negatively-tilted trough over New England late Thursday. By 12Z=20
Thursday, NAEFS shows 700mb and 500mb heights that are near the=20
lowest observed heights at 12Z for this time of year in the CFSR=20
climatology over Pennsylvania, indicating how anomalously deep=20
this cold-core low becomes. Moisture will stream northeast and
boundary layer temperatures look to remain sufficiently cold=20
enough to support snow as WAA this evening, especially in the=20
higher elevations of the Berkshires, Green, and White Mountains.=20
Expect moderate snow to begin over the southern Adirondacks this=20
afternoon and then across the rest of the northern Appalachians=20
this evening. Heavy snow will then envelope much of northern Maine
overnight. By Thursday morning, the surface low will track east
closer to Caribou, Maine with additional snowfall expected along=20
an inverted trough on the backside of the storm. Cyclonic flow=20
will allow for upslope snow to persist through Thursday night in=20
the northern Appalachians before tapering off Friday morning. WPC=20
PWPF continues to show high chances (>70%) for snowfall amounts >8"
in the southern Adirondacks and Greens, then along the White=20
Mountains up through central and northern Maine. Localized amounts=20
topping 12" are likely in the higher portions of these mountain=20
ranges.


2. Lake-Effect Snow (LES): In wake of the exceptional LES event=20
that occurred downwind of Lakes Erie and Ontario over the past=20
week, yet another round of heavy LES is on tap starting tonight=20
and lasting through Thursday night, then another round Friday=20
night. The cold front passes Lakes Erie and Ontario late tonight=20
with surface-850mb winds quickly changing to the west behind the=20
front and then northwesterly Thursday afternoon. Expect mostly=20
multi- banded segments off Lake Ontario, with CAMs agreeing on=20
longer single banded solutions emanating from Lake Huron and over=20
Erie Co PA. This band could not only bring heavy snow to the hard=20
hit Chautauqua Ridge in far western NY, but potentially into=20
central PA as well. Lake-effect snow should linger through Thursday
night and begin to taper off later in the day on Friday as winds=20
weaken over the Lakes. However, winds backing to westerly flow=20
Friday night should allow some single band formation off both=20
lakes. Latest Day WPC PWPF shows high chances (>70%) for snowfall=20
above 18" in northwest Pennsylvania along the Chautauqua Ridge,=20
where there are also low chances (10-30%) for snowfall above 24"=20
through Friday night. Along the Tug Hill Plateau with more like 50%
for the southern Adirondacks which is after the WAA snow through=20
this evening. WPC PWPF shows moderate chances (40-60%)=20
probabilities for >24" in the highest terrain of the Tug Hill=20
through Thursday night. Regionally, these snowfall amounts combined
with whipping wind gusts topping 40 mph will likely result in=20
significant blowing/drifting of not just the snow from this event,=20
but also residual snowpack from heavy snow that accumulated over=20
the past 4-5 days.

3. Snow Squalls/Mountain Snow: This strong cold front, which
according to some cross-section guidance shows FGEN that extend as
far up in the atmosphere as 500mb, will coincide beneath the left-
exit region of a 100kt 500mb jet streak. By 00Z tonight, 500mb=20
winds over Iowa are topping the maximum observed winds for this=20
time of year in the CFSR climatology. As the Arctic front pushes=20
south and east, the thermal gradient will not be as strong as it=20
could've been had the frontal passage occurred during the daytime=20
hours. That said, the aforementioned deep 500-700mb height=20
anomalies and strong synoptic-scale forcing will provide sufficient
lift, along with ample 850-700mb FGEN to foster formidable snow=20
squalls this evening from southern Michigan and northern IN on east
through the Upper Ohio Valley and into the northern Mid- Atlantic=20
overnight. These squalls are likely to traverse the Delaware Valley
on south to potentially northern MD and northern VA in the early=20
Thursday morning. Boundary layer temperatures are more questionable
along the I-95 corridor, but fast moving squalls producing brief=20
bursts of snow should occur in parts of the Piedmont regions of=20
northern VA and northern MD. Day 1.5 PWPF for >0.1" has a curious=20
maximum over the northern VA Piedmont. Remember there are no safe=20
places on a highway during a snow squall as visibility drops=20
significantly and roads can become snow covered quickly, making it=20
hard to see on the road and slow down when necessary.=20

4. Upslope Mountain Snow: Farther south, strengthening low level=20
westerlies will flow orthogonally to the SW-NE spine of the Central Appalachians. Strong vertical ascent not only via synoptic scale=20
and FGEN methods but also through orographic lift should prompt=20
bursts of heavy snow to continue after the frontal squalls late=20
tonight into Thursday with rapidly increasing SLRs. To make impacts
worse, 850mb winds over the Central Appalachians (including the=20
Laurel and Potomac Highlands) will be in excess of 50 kts, with the
strong winds leading to not only substantial blowing/drifting of=20
snow, but potential blizzard conditions that could even cause power
outages in some areas. WPC PWPF shows high chances (>70%) for >4"=20
of snowfall in the central Appalachians with parts of the Laurel
Highlands (far southwest PA, western Garrett County, MD, and south
of Garrett County in eastern WV) sporting moderate chances=20
(40-60%) for snowfall >8" through Friday evening.

...Washington Cascades...
Day 3...

The Pacific Northwest sees the return of some wintry weather on
Saturday as a Pacific storm system brings high elevation snow

4,000ft) to the region. Some moisture spilling over into the

Columbia Basin could also have boundary layer temps that are sub-
freezing on Saturday. Some minor ice accumulations are possible
within the Columbia Basin on Saturday, as are locally heavy=20
snowfall totals through Saturday night in the higher terrain

5,000ft) of the Cascades and Olympics through Saturday afternoon.

WPC PWPF shows low-to-moderate (30-50%) chances for >4" above
5,000ft through Saturday afternoon with potentially more high
elevation snowfall to come in the early onset of the medium range
period.=20


The probability of significant ice across the CONUS is less than=20
10 percent.=20


Mullinax




...Winter Storm Key Messages are in effect. Please see current=20
Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!-g1dxd4MpbdveXRjoRobaYfVXKYpEeZGu6OhPHsQBcBhj= ar9wEuNlo9ll3np0NWfCiGFyPHc5dJUsSWB98MOSTIMXyI$=20

$$

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FOUS11 KWBC 050629
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
129 AM EST Thu Dec 5 2024

Valid 12Z Thu Dec 05 2024 - 12Z Sun Dec 08 2024

...Eastern Great Lakes and Northeast...=20
Days 1-3...=20

Strong low pressure moving through Quebec this morning will=20
continue eastward, with WAA-driven snow over northern New England.=20
500mb heights/wind are near the CFSR reanalysis min/max=20
(respectively) with this potent system. The associated cold front,=20 responsible for numerous snow squalls yesterday afternoon and=20
overnight, will help feed moisture into northern/interior Maine=20
where sub-freezing temperatures will support all snow this morning.
Low pressure will quickly translate into Atlantic Canada early=20
this afternoon, with the FROPA turning winds around to the=20
northwest and helping to enhance orographic lift across much of the
Northeast into the central Appalachians. Strong pressure gradient=20
will also maintain windy conditions with blowing/drifting snow over
the interior and near-blizzard/blizzard conditions in the higher=20
elevations (esp WV). Lake-effect snow, already in progress over MI,
will redevelop/strengthen over the eastern Great Lakes by this=20
afternoon on NW flow with some double (Lake) dip increase in=20
moisture. CAM guidance continues to depict some rather robust=20
single bands off some of the lakes with a far reach inland of at=20
least lighter snow. Winds will eventually back and lighten by=20
tomorrow with less coverage/intensity overall but still some more=20
potent bands possible. A trailing shortwave out of Canada will=20
swing through the Lakes late Friday, again enhancing some lake-=20
effect snows across MI into NY and NW PA.=20

WPC probabilities for at least 12 inches of additional snow after=20
12Z Thu are highest (>50%) east of Lake Erie (NW PA into SW NY) and
southeast of Lake Ontario (between ART and SYR). Moderate snow=20

8") is likely over the higher terrain of the Adirondacks into the

Green and White Mountains due to upslope flow. In addition, the=20
eastern U.P. of Michigan and the western side of Lower Michigan=20
show moderate (40-70%) and low (10-40%) probabilities,=20
respectively, of at least another 8 inches of snow through=20
Saturday.=20

Lastly, with the progression of the cold front this morning, the=20
threat of snow squalls remains for parts of the northern Mid-=20
Atlantic into New England. This may produce a quick drop in=20
visibility coincident with gusty winds and slippery travel.=20


...Pacific Northwest/Northern Rockies...=20
Day 3...

Upper low in the northeastern Pacific will stream eastward along=20
50N into British Columbia, bringing in a surge of moisture to the=20
Cascades then into the northern Rockies on Saturday. Snow levels=20
will be on the higher side, about 8000ft at precip onset, but will=20
lower to around 5000-6000ft Saturday afternoon as the front moves=20
in, and falling farther to around 3500ft as the precipitation=20
lightens into Sunday morning. Snow will likely affect the higher=20
passes, where the probability of at least 6 inches is >50% above=20
4000ft or so. East of the Cascades, some icing and/or a wintry mix=20
of sleet/freezing rain will be possible as colder surface=20
temperatures are overrun by mild air aloft. Into the northern=20
Rockies, moisture will get wrung out over the higher terrain with=20
several inches likely (>50% chance of 6 inches) on the higher=20
mountain ridges.=20


The probability of significant ice >0.25" across the CONUS is less
than 10 percent.=20


Fracasso



...Winter Storm Key Messages are in effect. Please see current=20
Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!_SE21lAdIfWr2_E8riMb_I83_oj2-e2RtStvv7M3CA5zx= aP-iR8mkCa83hFy0rm25LoL5Zg-zgN9lFM2TYGyrW9ETZs$=20

$$

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FOUS11 KWBC 051942
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
242 PM EST Thu Dec 5 2024

Valid 00Z Fri Dec 06 2024 - 00Z Mon Dec 09 2024

...Great Lakes and Northeast...
Days 1-3...

Through the remainder of Thursday afternoon and into Friday, the
main story will be the ongoing lake-effect snow that will unfold
across parts the Great Lakes and even parts of the interior
Northeast down wind of Lakes Erie and Ontario. Through this
afternoon and evening, geopotential heights and temperatures at=20
850mb, 700mb, and 500mb all will all be below the 2.5=20
climatological percentile according to NAEFS. Combined with=20
daytime heating, look for occasionally potent snow showers (maybe=20
a squall or two) to be possible across New England. Meanwhile,=20
brisk cyclonic flow over the warm Great Lakes will keep LES=20
streamers in the forecast this afternoon and through tonight over=20
the eastern half of Michigan's U.P. and along the western half of=20
Michigan's Mitten. Farther east, strong NWrly flow paired with low-
level convergence will support some single-banded LES streamers=20
originating off Huron that poses a heavy snow threat in northwest=20
PA and far northeast OH. Lingering LES snow is also expected in=20
central NY as LES lingers tonight off of Lake Ontario.

By Friday, a 500mb vort max approaching from western Ontario allows
for some upper level divergence aloft that is also supported by the
northern Great Lakes position beneath the diffluent left-exit
region of a 125kt 250mb jet streak. Subtle low-level WAA is also
likely that will prompt the development of a stationary front that
acts as a trigger for additional snowfall through Saturday. This
upper level disturbance will revitalize the westerlies across Lakes
Erie and Ontario, resulting in the redevelopment of steady single
banded LES streamers that could produce 1-2"/hr snowfall rates
Friday evening and linger through Saturday morning. All told
through 12Z Saturday, much of Erie County, PA and south of Buffalo
in western NY (including along the I-90 corridor) have high=20
chances (>70%) for snowfall totals topping 18" with some localized=20
areas surpassing 24". This is also the case east of Lake Ontario=20
to the south of Watertown and along the Tug Hill. WSSI shows
Moderate to Major impacts in these areas, indicating the potential
for dangerous to even impossible travel conditions tonight and=20
into Saturday.

Meanwhile, the next in a series of upper level disturbances will=20
spawn a wave of low pressure that will track along the=20
aforementioned stalled frontal boundary Saturday afternoon to the=20
north of Lake Superior. This storm system will have strong=20
850-700mb WAA and isentropic ascent to work with, resulting in a=20
shield of snow from central NY on east through much of New England=20
by Sunday. WPC PWPF between 00Z Sunday - 00Z Monday shows low=20
chances (10-30%) for snowfall totals >4" in the peaks of the Adirondacks, Green, and White Mountains.


...Pacific Northwest/Northern Rockies...=20
Day 3...

An upper low in the northeastern Pacific heading into British
Columbia will direct two shortwave troughs into the Pacific
Northwest this weekend. Snow levels will initially be as high as
8,000ft Saturday morning, but fall to as low as 3,000ft by=20
Saturday night as a cold front pushes through the region. An=20
influx of Pacific moisture will push through Saturday afternoon in=20
the Pacific Northwest, then into the Northern Rockies by Saturday=20
night where a progressive precipitation shield will lead to some=20
minor snowfall accumulations. This same moisture plume will overrun
boundary layer temperatures that are sub-freezing, while mid-level
temperatures are above freezing. This setup likely leads to some
sleet/freezing rain mix in parts of the Columbia Basin where cold
air is harder to erode.

The next shortwave trough approaches from the northwest Saturday
night but this time with a better source of cold air in place=20
thanks to the previous cold frontal passage. Snow levels will
generally be around 3,000ft, allowing for those above 3,000ft in
the Cascades and Olympics to receive steady rounds of snow through
Sunday afternoon. Most snowfall in the Northern Rockies will be on
the minor side, although some totals topping 6" in the Lewis Range
are possible through Sunday. WPC PWPF shows high chances (>70%) for
snowfall topping 8" at elevations above 4,000ft, suggesting some
travel disruptions are possible at pass level in the Cascades this
weekend. Note however that the heaviest snowfall is likely to be
confined to elevations above 5,000ft in the Cascades. WPC PWPF=20
also shows moderate-to-high chances (50-70%) for snowfall >4" in=20
parts of the Bitterroots and Lewis Range that could result in some=20
locally treacherous road conditions above 5,000ft this weekend.=20
Lastly, there are low chances (10-40%) for minor ice accumulations

0.1" accretion) in localized parts of the Columbia Basin on=20

Saturday that could result in slick travel for motorists.


The probability of significant ice >0.25" across the CONUS is less
than 10 percent.=20

Mullinax




...Winter Storm Key Messages are in effect. Please see current=20
Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!7V1mjkJK88QCrkBHnSbkVhtzIgwzqw0UDfdzj-Xkr__CP= pQTyLTRkWQ26VUhqXuRiFMDQeKmJIAJUGSVDsOKcaI7LiI$=20

$$

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FOUS11 KWBC 060718
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
218 AM EST Fri Dec 6 2024

Valid 12Z Fri Dec 06 2024 - 12Z Mon Dec 09 2024

...Great Lakes and Northeast...
Days 1-2...

Departing area of low pressure over Atlantic Canada and modest
high pressure over the Mid-MS Valley will maintain cyclonic flow
across the Great Lakes, especially off Lakes Erie/Ontario, for the
next day or two. A shortwave within the broad trough over the
region will help back the flow enough to instigate another strong
single band off Ontario into the Tug Hill between Watertown and
Syracuse this afternoon/evening and overnight. Robust snowfall
rates of 1-3"/hr (per the HREF) will help deposit another 12-18"+
of snow over favored areas before waning on Saturday. Downwind of
Lake Erie, multi-band streamers will result in a larger area of
light to moderate snow with WPC probabilities of at least another
6" of snow >50% from Erie, PA northeastward to the Chautauqua Ridge
in NY.

Another shortwave rounding the base of the longwave trough will
slip through the Great Lakes late Sat into early Sun with generally
light snow for the U.P., northern Lower MI, into NYS and northern
New England. WPC probabilities for at least 4 inches of snow are
generally low (10-40%) except for areas with lake enhancement and
parts of the Adirondacks, Green, and White Mountains where
probabilities are moderate (40-70%).


...Pacific Northwest/Northern Rockies...
Days 2-3...

An upper low currently south of the Gulf of Alaska will move
eastward over the next day or so, aimed toward British Columbia.
Ahead of it, a plume of moisture will be directed into the Pacific
Northwest starting early Saturday. Snow levels will start quite
high (around 8000-9000ft at precip onset very early Saturday) but
fall to around pass level (~4000ft) late Saturday into the
overnight hours. East of the crest, cold air in place at the
surface with overrunning WAA-driven precip supports a wintry mix of snow/sleet/freezing rain for eastern WA. Some icing is possible,
from a few hundredths to around a tenth of an inch.

A second surge in moisture will occur on Sunday associated with
the parent upper low as it opens up across the region. With snow
levels already around pass level, snow should accumulate on Sunday
with WPC probabilities for at least 6 inches of snow >50% above
about 3500ft or so.

Farther east, moisture will spread across northeastern OR and into
the northern Rockies from late Saturday through the end of the
period early Monday. Snow levels will lower through time, and
nearly all areas except the lowest valleys will change from rain to
snow. WPC probabilities for at least 8 inches of snow days 2-3 are

50% above about 5000ft or so, mostly in the Lewis Range and

across northern ID, the Blue Mountains, and into northwestern WY.


...Northern Minnesota and northeastern North Dakota...
Day 3...

Eastern extension of the northern Rockies system will manifest
itself as another upper low forming by Sunday over southern
Saskatchewan. That will move east-southeastward into early Monday
with WAA-driven precipitation to its east from northeastern ND
eastward across northern MN. Ptype will be an issue with milder air
overtopping the colder boundary layer, especially where precip
arrives near/after sunset Sunday. NBM ptype uncertainty maximizes
between all 4 types in a zone along the U.S.-Canadian border though
the majority of probabilities are either rain or snow. Still,
cannot rule out some light icing but will have time to narrow the
forecast with time. WPC probabilities for at least 4 inches of snow
are moderate (40-70%) over northern MN.


The probability of significant ice >0.25" across the CONUS is less
than 10 percent.

Fracasso


$$

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FOUS11 KWBC 061936
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
236 PM EST Fri Dec 6 2024

Valid 12Z Fri Dec 06 2024 - 12Z Mon Dec 09 2024

...Great Lakes and Northeast...
Days 1-2...

Lake-effect snow (LES) bands will continue throughout portions
of the northern and eastern Great Lakes today as high pressure to
the south and a large area of low pressure over Atlantic Canada act
to sustain cyclonic flow over the Lakes. By this evening, LES snow
in northern Michigan and the Michigan U.P. will taper off while
LES bands will become reinvigorated down-wind of Lakes Erie and
Ontario. This is due to an approaching shortwave trough that both
backs and accelerates westerly low-level winds over these Lakes
overnight. These bands will pack a punch as 1-3"/hr snowfall rates
are likely in these single bands that are expected to produce
another 12-18" (locally up to 24" possible) from the eastern shores
of Lake Ontario on east to the I-81 corridor between Watertown and
Syracuse and finally to the Tug Hill. Farther south, it will be
areas along I-80 in northwest PA and ENE along the Chautauqua Ride
that could receive 8-12" of snow with localized amounts
approaching 18". LES snow bands should gradually taper off
Saturday morning as WAA and WSWrly winds pick up.

While these LES bands diminish, another strong system will race
east across southern Canadian Provinces Saturday morning with
sufficient 850-700mb WAA along the storm's warm front leading to a
progressive shield of snow moving east across the northern Great
Lakes. By Saturday evening, the warm front advances through the
Northeast with periods of snow enveloping parts of central and
northern NY to start, then into northern New England Saturday night
and into Sunday morning. The storm is rather progressive and a dry
slot will quickly dry out the column by late Sunday morning.
However, some minor accumulations are possible, especially in the
Adirondacks, Green, and White Mountains. WPC PWPF shows moderate
chances (40-60%) for snowfall totals topping 4" in these
mountains. One area to monitor is Downeast Maine, for if the storm
can quickly wrap around a comma-head of precipitation Sunday
afternoon, there could be localized snowfall totals that approach
4" in some spots. The WSSI-P does show similar moderate
probabilities (40-60%) for Minor Impacts in Downeast Maine on
Sunday.


...Pacific Northwest/Northern Rockies...
Days 1-3...

An upper low in the northeast Pacific heading into British Columbia
will direct two shortwave troughs into the Pacific Northwest this
weekend. Snow levels will initially start out between
8,000-9,000ft Saturday morning, but plummet to as low as 3,000ft by
early Sunday morning. This will result in rain being the primary
precip type a the onset Saturday, but as the cold front moves
through and snow level drop, passes should see the changeover to
snow occur Saturday night. East of the Cascades, lingering sub-
freezing temperatures within the boundary layer will be trapped in
the Columbia Basin as WAA and overrunning moisture aloft provide a
favorable setup for an icy wintry mix in central and northeastern
Washington on Saturday. Some minor icing accumulations are possible
with areas west of I-97 in northern Washington having the better
odds for ice accumulations up to a tenth of an inch. Slick travel
conditions are possible in these areas on Saturday.

The second shortwave trough makes its way through the Pacific
Northwest Saturday night and into Sunday. Unlike the first event,
snow levels are now hovering at or just below pass level, making
this event the more concerning for potential travel impacts. The
good news is it is a quick moving system and snow should taper off
by Sunday night as high pressure builds in by the start of next
week. WPC PWPF does show moderate-ot-high probabilities (50-70%)
for >6" of snow for the duration of this weekend event in the
Snoqualmie Pass, while there are high chances (>70%) for snowfall
totals >12" at elevations above 5,000ft in the Cascades and
Olympics.

These atmospheric disturbances will push their respective plumes
of moisture east into the Northern Rockies as well, with the
heaviest snow ensuing Saturday night and into Sunday as snow levels
decrease, SLRs increase, and northerly low level winds aid in
upslope enhancement for snowfall rates. For northern Idaho snowfall
totals of 6-12" are possible above 5,000ft in the Bitterroots. The

6,000ft elevations of the Lewis Range, Blue, and Bitterroot

Mountains in central Idaho will be favored for similar totals.
Lastly, parts of the higher terrain of the Little Belt Mountains in
central Montana, the Absaroka in southern Montana, and the Tetons
of western Wyoming could also see 6-12" of snow, particularly above
7,000ft.

...Northern Minnesota and northeastern North Dakota...
Days 2-3...

The first upper level shortwave trough that brought snow to parts
of the Pacific Northwest mountain ranges will spawn low pressure
over the Canadian Prairies Sunday morning. As its warm front moves
east, WAA across the Northern Plains will result in a shield of
precipitation that passes over a sub-freezing boundary layer.
While precipitation may start out as snow through as the
atmospheric column cools to a sub-freezing wet-bulb temperature
profile, the strong WAA will cause low level temps (850-750mb) to
rise above freezing and thus cause snow to change over to a
sleet/freezing rain mix from the Red River of the North on east
into northern Minnesota. It should be noted that this can still
change, for by Sunday night, the primary occluded low tracking
through southern Saskatchewan and Manitoba will weaken as a new low
forms along the triple point in the Upper Mississippi Valley.
Should this low form sooner and or track farther south, it could
mean snow occurs for a longer duration across northern Minnesota
either due to the storm track, or a deeper low Sunday night that
allows for a more defined deformation axis on its northern flank.
These solutions are depicted in some members of the WPC super
ensemble and are largely why WPC PWPF shows moderate chances
(40-60%) for snowfall greater than 4" along the northern Minnesota
border and into the northern most section of Minnesota's
Arrowhead. Residents in North Dakota and northern Minnesota should
follow the forecast closely in the coming days as the forecast
comes into better focus over the next 24-36 hours.


The probability of significant ice >0.25" across the CONUS is less
than 10 percent.

Mullinax



$$

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FOUS11 KWBC 070727
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
227 AM EST Sat Dec 7 2024

Valid 12Z Sat Dec 07 2024 - 12Z Tue Dec 10 2024

...Great Lakes and Northeast...
Day 1...

Shortwave on the southwest side of the expansive eastern North
American trough will zip quickly across the Great Lakes and
into/through the Northeast later today. With a surface low track
through southern Ontario, WAA-driven snow will spread across the
Northeast and end early Sunday. With a general westerly wind
component, upslope into N-S terrain (e.g., the Green and White
Mountains) will promote moderate amounts of snow with lighter
amounts in lower elevations. Some additional lake enhancement is
also likely off Lake Ontario into the Tug Hill. Quick movement of
the system should limit amounts, and WPC probabilities for at least
4 inches of snow are highest (>50%) north of I-90 in NY (Tug
Hill/Adirondacks) and into the Green and White Mountains. By later
Saturday night into early Sunday, as the system departs, could see
some late enhancement over eastern Maine where WPC probs for 4
inches are also near 50%.


...Pacific Northwest/Northern Rockies...
Days 1-3...

An upper low in the northeast Pacific heading into British
Columbia will direct two shortwave troughs into the Pacific
Northwest this weekend. Snow levels will initially start out
between 8,000-9,000ft this morning, but plummet to as low as
3,000ft by early Sunday morning. Rain at most locations will change
to snow at Stevens/Snoqualmie Passes as the first wave of
precipitation wanes. East of the Cascades, lingering sub-freezing
temperatures within the boundary layer will be trapped in the
Columbia Basin as WAA and overrunning moisture aloft provide a
favorable setup for an icy wintry mix in central and northeastern
Washington today and even into parts of southeastern
OR/southwestern ID tonight. Some minor icing accumulations are
possible up to about a tenth of an inch.

The second shortwave trough makes its way through the Pacific
Northwest tonight and into Sunday. Lower snow levels support
moderate snow at pass level (esp Stevens) with this second wave of
moisture. WPC probabilities for at least 8 inches of snow are
highest above about 3500ft.

These atmospheric disturbances will push their respective plumes
of moisture east into the Northern Rockies as well, with the
heaviest snow tonight and into Sunday as snow levels decrease, SLRs
increase, and northerly low level winds aid in upslope enhancement
for snowfall rates. Highest probabilities for >6" snow are over
the terrain including the Bitterroots, Lewis Range, Blue Mountains,
and into the Little Belt Mountains in central Montana, the
Absarokas in southern Montana, and the Tetons of western Wyoming.


...Northern Minnesota and northeastern North Dakota...
Days 2-3...

The first upper level shortwave trough that brought snow to parts
of the Pacific Northwest mountain ranges will spawn low pressure
over the Canadian Prairies Sunday morning. As its warm front moves
east, WAA across the Northern Plains will result in a shield of
precipitation that passes over a sub-freezing boundary layer. While precipitation may start out as snow along the Canadian border,
mid/low-level WAA may promote an area of freezing rain for portions
of eastern ND into MN late tonight into early Sunday. Depending on
the evolution of the main low along the Canadian border and other
frontal waviness over the Corn Belt moving up toward the MN
Arrowhead, snow could be maximized over northeastern MN from north
of Duluth to the border. WPC probabilities for at least 4 inches of
snow are >50% from around the Northwest Angle southeastward to
Embarrass and points northeast.


...CO Rockies/Sangre de Cristos...
Day 3...

Height falls associated with the second wave through the Pacific
Northwest will push through the central Rockies, spreading light
snow across the region on Monday into early Tuesday. Quick movement
and general lack of moisture will limit snowfall, with WPC
probabilities of at least 4 inches >30% limited to the Sangre de
Cristos in CO and just into NM.


The probability of significant ice >0.25" across the CONUS is less
than 10 percent.

Fracasso/Mullinax






$$

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FOUS11 KWBC 071933
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
233 PM EST Sat Dec 7 2024

Valid 12Z Sat Dec 07 2024 - 12Z Tue Dec 10 2024

...Northeast...
Day 1 & Day 3...

Some additional lake enhancement is possible for a few more hours
downwind of Lake Ontario into the Tug Hill through mid-afternoon.
A storm system passing through the northern Great Lakes will track
quickly through southern Ontario tonight and into the Northeast by
early Sunday morning. With WAA and isentropic ascent being the
primary sources of lift, along with a progressive plume of
850-700mb moisture, snow will spread across the Northeast this
afternoon and conclude by Sunday morning as the storm heads for
Nova Scotia. The warm nose aloft caused by the low-level WAA may
lead to light ice accumulations in parts of the the interior
Northeast tonight and into Sunday morning. With the mean wind flow
more out of the west, upslope into N-S terrain (the Green and White
Mountains most notably) will promote moderate amounts of snow with
lighter amounts in lower elevations (coastal and valleys). This
storm progressive nature will limit amounts, as evident by WPC
probabilities for at least 4 inches of snow are highest (>70%)
north of I-90 in NY (Tug Hill/Adirondacks) and into the Green and
White Mountains. It is in these higher elevations (above 2,000ft)
where WPC probabilities have moderate chances (40-60%) for snowfall
totals above 6 inches. Note that Downeast Maine could also see
some minor snow accumulations as WPC probs depict moderate chances
(40-60%) for above 4 inches of snowfall through midday Sunday.

The next winter storm to affect the region comes in the form of low
pressure tracking through the Southeast that is accompanied by a
Gulf of Mexico moisture plume. Weak high pressure is located over
Quebec that will aid in the development of a cold-air-damming
signature as 850-700mb WAA out of the Southeast and subsequent
isentropic glide leads to snow being the primary precipitation type
over New England at the onset Monday afternoon. However, as WAA
increases, the burgeoning >0C warm nose aloft will force p-types to
change to a wintry mix in parts of central New England. This too
will be a progressive storm system, so snowfall totals are not
expected to be overly heavy. That said, WPC PWPF shows low chances
(10-30%) for minor snowfall accumulations of 2 inches or more in
parts of northern Maine on south to the Green and White Mountains.
These areas could also see minor ice accumulations Monday night
into Tuesday morning as well that could result in slick travel
conditions in parts of northern New England on Tuesday morning.


...Pacific Northwest/Northern Rockies...
Days 1-2...

The first of two shortwave troughs approaching the Pacific
Northwest is producing periods of high elevation mountain snow at
the moment, but snow levels will plummet to as low as 3,000ft later
tonight. Rain will change to snow at Stevens/Snoqualmie Passes
later this afternoon but snowfall rates should gradually diminish
at pass level by this evening. East of the Cascades, lingering
sub-freezing temperatures within the boundary layer are initially
trapped in the Columbia Basin as WAA and overrunning moisture aloft
provide a favorable setup for an icy wintry mix in central and
northeastern Washington today and even into parts of southeastern OR/southwestern ID through this evening. Some minor icing
accumulations are possible up to about a tenth of an inch.

The second shortwave trough arrives tonight with periods of
mountain snow lingering into Sunday. Lower snow levels, compared
to the start of the first shortwave trough's approach on Saturday,
support moderate snow at pass level (especially Stevens Pass) with
this second wave of moisture. WPC probabilities sport high chances

70%) for at least 8 inches of snow above about 3,500ft while the

more remote areas above 5,000ft likely receive over 12" of
snowfall through Sunday evening. Snow should taper off by Sunday
afternoon as high pressure builds in from the West, thus allowing
for a more tranquil start to the work-week in the Pacific
Northwest.

Both atmospheric disturbances will push their respective plumes of
moisture east into the Northern Rockies with the heaviest snow
tonight and into Sunday as snow levels decrease, SLRs increase, and
northerly low level winds aid in upslope enhancement for snowfall
rates. Highest probabilities for >8" of snow are over the higher terrain including the Bitterroots, Lewis Range, Blue Mountains, and into
the Little Belt Mountains in central Montana, the Absarokas in
southern Montana, and the Tetons of western Wyoming.


...Northern Minnesota and northeastern North Dakota...
Days 1-2...

The first upper level shortwave trough that brought snow to parts
of the Pacific Northwest mountain ranges on Saturday will spawn
low pressure in the Canadian Prairies Saturday night. As its warm
front moves east, WAA across the Northern Plains will produce a
shield of precipitation that passes over a sub-freezing boundary
layer. While precipitation may start out as snow along the Canadian
border, mid/low-level WAA may promote an area of freezing rain for
portions of eastern North Dakota into Minnesota late tonight into
early Sunday. WPC probabilities do show some low chance
probabilities (10-30%) for >0.1" of ice accumulations in parts of
the Red River of the North and north-central Minnesota early Sunday
morning. In terms of snowfall, latest guidance supports the
heaviest snow occurring with the initial WAA over northern
Minnesota, and more specifically in the Minnesota Arrowhead. WPC
probabilities for at least 4 inches of snow are >50% from far
northeast North Dakota to much of northern Minnesota. The Minnesota
Arrowhead has moderate-to-high chances (50-70%) for >6" of
snowfall through Sunday evening. The WSSI-P shows moderate
probabilities (40-60%) for Minor Impacts (or winter driving
conditions that suggests some hazardous travel conditions) in far
northern Minnesota.


...CO Rockies/Sangre de Cristos...
Days 2-3...

Height falls associated with an elongated 500mb vorticity max and
a plume of residual Pacific moisture connected with the second
wave through the Pacific Northwest will push through the central
Rockies on Monday. High pressure building in from the north in wake
of a cold frontal passage Monday morning will result in some weak
upslope flow into Colorado's Front Range during the day. This
upslope component will extend as far south as the Sangre De Cristo
in northern New Mexico by Monday night. Periods of light-to-
moderate snow across the region on will ensue Monday into early
Tuesday. The progressive movement of the upper level disturbance
combined with the aforementioned atmospheric moisture plume being
gradually drying out through Tuesday morning will limit snowfall
totals. WPC probabilities show moderate probabilities (40-60%) of
at least 4 inches mainly limited to Sangre de Cristos in southern
Colorado and northern New Mexico, which does include the Raton
Mesa. The highest and more remote elevations could see localized
amounts approach 8" by the time the snow comes to an end Tuesday
morning.


The probability of significant ice >0.25" across the CONUS is less
than 10 percent.

Mullinax




$$

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FOUS11 KWBC 080812
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
312 AM EST Sun Dec 8 2024

Valid 12Z Sun Dec 08 2024 - 12Z Wed Dec 11 2024

...Northeast...
Day 1...

Quick-moving system crossing from Ontario into Quebec before 12Z
will continue eastward across northern Maine this morning and early
afternoon before moving into Atlantic Canada. Light snow is
forecast after 12Z this morning for the Adirondacks and northern
New England, with a bit higher amounts over eastern Maine where
snow will eventually taper off later this afternoon. WPC
probabilities for at least 4 inches of snow today after 12Z are
20-40% over far eastern Maine.

Day 2...

The next winter storm to affect the region comes from a
combination of a shortwave over the Ohio Valley and a northern
stream upper low over southern Canada, both acting in concert to
coax a moisture plume northward from the Gulf of Mexico on WAA. A
mix of some freezing rain on the southern side (northeast PA
northward through NYS and also over parts of western New England)
and snow on the northern side (north of I-90) may cause some minor
impacts on Monday. WPC probabilities for at least 4 inches of snow
are not higher than 10 percent, but are moderate (40-70%) for at
least 2 inches over northern NH into western ME.

Day 3... Yet another system will start to affect the Northeast as
the pattern becomes more amplified in the East. Through 12Z Thu,
strong WAA should scour out most of the cold air over the region,
but interior Maine may see some light icing very early Wednesday
with much more precipitation into D4.


...Pacific Northwest/Northern Rockies...
Days 1-2...

Second part of the two-part system will affect the NW today,
pushing moisture into a much colder environment. Moderate to heavy
snow at the higher passes may continue to cause moderate impacts
this morning before the snow starts to taper off from west to east
this afternoon. WPC probabilities depict high chances (>70%) for at
least 8 inches of snow above about 3,500ft while the more remote
areas above 5,000ft likely receive an additional 12"+ of snowfall.

Moisture will continue east of the Cascades across to the Rockies
as the cold front has moved well past the Divide. WPC probabilities
for >8" of snow are over the higher terrain including the
Bitterroots, Lewis Range, Blue Mountains, and into the Little Belt
Mountains in central Montana, the Absarokas in southern Montana,
and the Tetons of western Wyoming. Snow will continue into D2
farther east over the Black Hills where probabilities of at least 4
inches reach 60-90%.


...Northern Minnesota and northeastern North Dakota...
Day 1...

Low pressure over southern Saskatchewan this morning will move ESE
toward northern ND/MN this evening and overnight, lifting its warm
front through the region. While precipitation may start out as
snow along the Canadian border, mid/low-level WAA may promote an
area of freezing rain for portions of eastern North Dakota into
Minnesota this morning, with several hundredths of an inch of ice
possible after 12Z. Closer to the Canadian border, a colder column
supports light to modest snowfall, with the heaviest snow occurring
with the initial WAA over northern Minnesota (esp the Arrowhead,
which will be enhanced by easterly flow off Lake Superior). WPC
probabilities for at least 4 inches of snow are >50% across much of
the northern part of MN, with higher probabilities over the
Arrowhead (>80%). There, probabilities for at least 8 inches of
snow are moderate (40-70%).


...CO Rockies/Sangre de Cristos...
Day 2...

Height falls at the southwest side of a large positively-tilted
longwave trough over the middle of North America will swing through
the central Rockies on Monday, brining with it a plume of residual
Pacific moisture. High pressure building in from the north in the
wake of a cold frontal passage Monday morning will result in some
weak upslope flow into Colorado's Front Range during the day. This
upslope component will extend as far south as the Sangre de Cristos
in northern New Mexico by Monday night. Periods of light-to-
moderate snow across the region are expected as the system moves
quickly through the region and heights build in by Tuesday. WPC
probabilities show >50% probabilities for at least 4 inches mainly
limited to the Sangre de Cristos in southern Colorado and northern
New Mexico, which does include the Raton Mesa. The highest and more
remote elevations could see localized amounts approach 8" by the
time the snow comes to an end Tuesday morning.


The probability of significant ice >0.25" across the CONUS is less
than 10 percent.


Fracasso/Mullinax







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FOUS11 KWBC 081958
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
258 PM EST Sun Dec 8 2024

Valid 00Z Mon Dec 09 2024 - 00Z Thu Dec 12 2024

...Northeast...
Days 1-3...

As the snow today in parts of the Northeast winds down, focus
shifts to the next wintry setup that to affect the region beginning
midday Monday, peaking in intensity Monday evening, and concluding
Tuesday morning. An upper level disturbance in the Ohio Valley
will direct a plume of rich Gulf of Mexico moisture towards the
region Monday afternoon. NAEFS shows IVT values of 300-400 kg/m/s
approaching the Northeast at 18Z. These IVT values are above the
90th climatological percentile according to NAEFS, highlighting the
anomalous nature of this moisture plume and advection. Meanwhile,
a storm system in the northern Great Lakes will work in tandem with
the Ohio Valley disturbance to produce brisk 700-300mb WSWrly
winds aloft. There is weak high pressure located over Quebec that
will help to lock in some low-level cold air and allow for
precipitation to fall in the form of snow Monday afternoon over
northern New England. However, the strong WSW flow aloft will force
a >0C warm nose to ensue at low levels and cause precipitation to
transition to an icy wintry mix for most areas.

By Monday evening, a weak coastal low near the tip of Long Island
that will then track towards the Massachusetts Capes. This low is
expected to introduce additional low-level moisture into Maine late
Monday night and into Tuesday. This could make for a more prolonged
period of snow or icy wintry mix, with the latter increasingly
more likely early Tuesday morning due to the dryslot aloft making
it increasingly difficult to favor dendrites. By the time the
storm departs for Nova Scotia around midday and precipitation
tapers off, snowfall amounts will generally range in the 1-3" range
for the Adirondacks, Green and White Mountains above 2,000ft.
Minor ice accumulations (generally topping out around 0.1") are
likely in many of these mountain ranges, as well as near the Maine
coast and as far south as the Poconos and Catskills.

By Wednesday morning a far more amplified jet stream pattern will
bring about the next chance for measurable snow, but there remains
a good deal of uncertainty in its evolution. NAEFS and ECMWF are in
good agreement on an anomalous upper level trough over the
Mississippi Valley 12Z Wednesday, but depth/tilt of the primary
500mb shortwave at the base of the upper level trough differs by
guidance members. The ECMWF ENS is a bit deeper with this southern
disturbance while the GEFS is stronger with the upper low
traversing southern Canada. This can have implications on the
transition from rain to snow throughout the Northeast and how
quickly rain can changeover to snow in the central Appalachians.
There are some aspects ensembles do agree upon; a lack of
sufficiently colder air (albeith enough to where parts of northern
Maine could witness up to a tenth of an inch of ice), a storm
track over the I-95 corridor, and lower SLRs that would support a
heavy/wet snow type. WPC PWPF does show the potential for some ice accumulations This kind of setup typically favors the Appalachians
for potential heavy snowfall, but when this transition to snow
occurs will be key in determining amounts and impacts in subsequent
forecast cycles.


...Northern Rockies & Northern High Plains...
Day 1...

Snow will taper off later this evening in the Olympics and Cascades
as the upper level trough responsible for the disruptive snowfall
amounts heads east. Pacific moisture associated with the upper
trough will continue east of the Cascades across to the Rockies as
the cold front advances through the High Plains and Central
Rockies. WPC probabilities show moderate-to-high chances (50-70%) for
snowfall >4" of snow are over the higher terrain including the
Bitterroots, Lewis Range, Blue Mountains, and into the Little Belt
and Big Snowy Mountains in central Montana, the Absarokas in
southern Montana, and the Big Horns in northern Wyoming.

As a storm system tracks east along the US/Canada border, a
deformation band of snow will pivot over eastern Montana and
western North Dakota this evening. The storm's progressive movement
and strong winds fracturing dendrites will put a cap on snowfall
totals, but those same winds are likely to cause blizzard
conditions in parts of eastern Montana. The WSSI does suggest
Moderate Impacts that are primarily driven by wind gusts that could
approach 55 mph, leading to whiteout conditions and snow drifts.
Strong NWrly winds also favor upsloping flow into the Black Hills
where WPC PWPF shows moderate-to-high chances for snow totals >4"
above 6,000ft. The strong upper level ascent and steep lapse rates
could support possible snow squalls tonight and into Monday
morning as well across the western Dakotas. Snow should taper off
throughout the region by Monday afternoon.


...Northern Minnesota and northeastern North Dakota...
Day 1...

Low pressure over southern Saskatchewan will move toward northern
ND/MN tonight, lifting its warm front through the region and acting
as the primary source of lift for precipitation throughout the
region. While precipitation will start out as snow along the
Canadian border, persistent mid/low-level WAA may cause snow to
changeover to freezing rain for portions of eastern North Dakota
and north-central Minnesota this morning, with several hundredths
of an inch of ice possible in parts of north-central Minnesota and
as far east as northern Wisconsin. The heaviest snow is set to
occur along the Minnesota Arrowhead where the initial snowfall via
WAA aloft will stay snow the longest, and snowfall rates tonight
will be enhanced by easterly flow off Lake Superior. WPC
probabilities for at least 6 inches of snow are >50% along the
northern coast of the Minnesota Arrowhead. In fact, the very tip of
the Arrowhead sports moderate chances (40-60%) for snowfall totals
over 12" through Monday afternoon.


...CO Rockies/Sangre de Cristos...
Days 1-2...

Height falls at the southwest side of a large positively-tilted
longwave trough over the Heartland will swing through the central
Rockies on Monday, bringing with it a plume of residual Pacific
moisture. High pressure building in from the north in the wake of a
cold frontal passage Monday morning will result in modest upslope
flow into Colorado's Front Range during the day. This upslope
component will extend as far south as the Sangre de Cristos in
northern New Mexico by Monday night. Periods of snow across the
region are expected as the system moves quickly through the region
and heights build in by Tuesday. WPC probabilities have increased
over the past 24 hours withj high chance probabilities (>70%) for
snowfall >4" that now stretchs from parts of the Palmer Divide all
the way to Raton Pass. The highest and more remote elevations could
see localized amounts approach 8" by the time the snow comes to an
end Tuesday morning, and this does include the Raton Pass where
snow could cause hazardous travel conditions.

...Great Lakes...
Day 3...

As a robust 500mb low dives south Wednesday morning, an Arctic
front and blusterly NWrly winds will race over the Great Lakes
starting Wednesday morning over Lake Superior, then over Lake
Michigan by Wednesday evening. NAEFS and ECMWF SATs agree that
850mb temps rushing over Lake Superior and Michigan will be below
the 10th climatological percentile. The sharp contrast between the
Lakes water temperatures and frigid low-level temperatures advected
by brisk NW winds will kick-start the lake-effect snow (LES)
machine into high gear yet again by Wednesday night. This same cold
air-mass will race over the eastern Great Lakes Wednesday night
and lead to more heavy snowfall into Thursday. For the scope of
this discussion (ending 00Z Thurs), WPC PWPF shows high chance
probabilities (>70%) for snowfall totals >6" along the northern
coasts of the Michigan U.P. and along the western coast of
Michigan's Mitten. Look for some snowfall rates within the more
robust bands to top >2"/hr Wednesday night in these areas.


The probability of significant ice >0.25" across the CONUS is less
than 10 percent.


Mullinax





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FOUS11 KWBC 082000
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
300 PM EST Sun Dec 8 2024

Valid 00Z Mon Dec 09 2024 - 00Z Thu Dec 12 2024

...Northeast...
Days 1-3...

As the snow today in parts of the Northeast winds down, focus
shifts to the next wintry setup that to affect the region beginning
midday Monday, peaking in intensity Monday evening, and concluding
Tuesday morning. An upper level disturbance in the Ohio Valley
will direct a plume of rich Gulf of Mexico moisture towards the
region Monday afternoon. NAEFS shows IVT values of 300-400 kg/m/s
approaching the Northeast at 18Z. These IVT values are above the
90th climatological percentile according to NAEFS, highlighting the
anomalous nature of this moisture plume and advection. Meanwhile,
a storm system in the northern Great Lakes will work in tandem with
the Ohio Valley disturbance to produce brisk 700-300mb WSWrly
winds aloft. There is weak high pressure located over Quebec that
will help to lock in some low-level cold air and allow for
precipitation to fall in the form of snow Monday afternoon over
northern New England. However, the strong WSW flow aloft will force
a >0C warm nose to ensue at low levels and cause precipitation to
transition to an icy wintry mix for most areas.

By Monday evening, a weak coastal low near the tip of Long Island
that will then track towards the Massachusetts Capes. This low is
expected to introduce additional low-level moisture into Maine late
Monday night and into Tuesday. This could make for a more prolonged
period of snow or icy wintry mix, with the latter increasingly
more likely early Tuesday morning due to the dryslot aloft making
it increasingly difficult to favor dendrites. By the time the
storm departs for Nova Scotia around midday and precipitation
tapers off, snowfall amounts will generally range in the 1-3" range
for the Adirondacks, Green and White Mountains above 2,000ft.
Minor ice accumulations (generally topping out around 0.1") are
likely in many of these mountain ranges, as well as near the Maine
coast and as far south as the Poconos and Catskills.

By Wednesday morning a far more amplified jet stream pattern will
bring about the next chance for measurable snow, but there remains
a good deal of uncertainty in its evolution. NAEFS and ECMWF are in
good agreement on an anomalous upper level trough over the
Mississippi Valley 12Z Wednesday, but depth/tilt of the primary
500mb shortwave at the base of the upper level trough differs by
guidance members. The ECMWF ENS is a bit deeper with this southern
disturbance while the GEFS is stronger with the upper low
traversing southern Canada. This can have implications on the
transition from rain to snow throughout the Northeast and how
quickly rain can changeover to snow in the central Appalachians.
There are some aspects ensembles do agree upon; a lack of
sufficiently colder air (albeit enough to where parts of northern
Maine could witness up to a tenth of an inch of ice), a storm track
over the I-95 corridor, and lower SLRs that would support a
heavy/wet snow type. WPC PWPF does show the potential for some ice accumulations This kind of setup typically favors the Appalachians
for potential heavy snowfall, but when this transition to snow
occurs will be key in determining amounts and impacts in subsequent
forecast cycles.


...Northern Rockies & Northern High Plains...
Day 1...

Snow will taper off later this evening in the Olympics and Cascades
as the upper level trough responsible for the disruptive snowfall
amounts heads east. Pacific moisture associated with the upper
trough will continue east of the Cascades across to the Rockies as
the cold front advances through the High Plains and Central
Rockies. WPC probabilities show moderate-to-high chances (50-70%) for
snowfall >4" of snow are over the higher terrain including the
Bitterroots, Lewis Range, Blue Mountains, and into the Little Belt
and Big Snowy Mountains in central Montana, the Absarokas in
southern Montana, and the Big Horns in northern Wyoming.

As a storm system tracks east along the US/Canada border, a
deformation band of snow will pivot over eastern Montana and
western North Dakota this evening. The storm's progressive movement
and strong winds fracturing dendrites will put a cap on snowfall
totals, but those same winds are likely to cause blizzard
conditions in parts of eastern Montana. The WSSI does suggest
Moderate Impacts that are primarily driven by wind gusts that could
approach 55 mph, leading to whiteout conditions and snow drifts.
Strong NWrly winds also favor upsloping flow into the Black Hills
where WPC PWPF shows moderate-to-high chances for snow totals >4"
above 6,000ft. The strong upper level ascent and steep lapse rates
could support possible snow squalls tonight and into Monday
morning as well across the western Dakotas. Snow should taper off
throughout the region by Monday afternoon.


...Northern Minnesota and northeastern North Dakota...
Day 1...

Low pressure over southern Saskatchewan will move toward northern
ND/MN tonight, lifting its warm front through the region and acting
as the primary source of lift for precipitation throughout the
region. While precipitation will start out as snow along the
Canadian border, persistent mid/low-level WAA may cause snow to
changeover to freezing rain for portions of eastern North Dakota
and north-central Minnesota this morning, with several hundredths
of an inch of ice possible in parts of north-central Minnesota and
as far east as northern Wisconsin. The heaviest snow is set to
occur along the Minnesota Arrowhead where the initial snowfall via
WAA aloft will stay snow the longest, and snowfall rates tonight
will be enhanced by easterly flow off Lake Superior. WPC
probabilities for at least 6 inches of snow are >50% along the
northern coast of the Minnesota Arrowhead. In fact, the very tip of
the Arrowhead sports moderate chances (40-60%) for snowfall totals
over 12" through Monday afternoon.


...CO Rockies/Sangre de Cristos...
Days 1-2...

Height falls at the southwest side of a large positively-tilted
longwave trough over the Heartland will swing through the central
Rockies on Monday, bringing with it a plume of residual Pacific
moisture. High pressure building in from the north in the wake of a
cold frontal passage Monday morning will result in modest upslope
flow into Colorado's Front Range during the day. This upslope
component will extend as far south as the Sangre de Cristos in
northern New Mexico by Monday night. Periods of snow across the
region are expected as the system moves quickly through the region
and heights build in by Tuesday. WPC probabilities have increased
over the past 24 hours with high chance probabilities (>70%) for
snowfall >4" that now stretches from parts of the Palmer Divide
all the way to Raton Pass. The highest and more remote elevations
could see localized amounts approach 8" by the time the snow comes
to an end Tuesday morning, and this does include the Raton Pass
where snow could cause hazardous travel conditions.

...Great Lakes...
Day 3...

As a robust 500mb low dives south Wednesday morning, an Arctic
front and blustery NWrly winds will race over the Great Lakes
starting Wednesday morning over Lake Superior, then over Lake
Michigan by Wednesday evening. NAEFS and ECMWF SATs agree that
850mb temps rushing over Lake Superior and Michigan will be below
the 10th climatological percentile. The sharp contrast between the
Lakes water temperatures and frigid low-level temperatures advected
by brisk NW winds will kick-start the lake-effect snow (LES)
machine into high gear yet again by Wednesday night. This same cold
air-mass will race over the eastern Great Lakes Wednesday night
and lead to more heavy snowfall into Thursday. For the scope of
this discussion (ending 00Z Thurs), WPC PWPF shows high chance
probabilities (>70%) for snowfall totals >6" along the northern
coasts of the Michigan U.P. and along the western coast of
Michigan's Mitten. Look for some snowfall rates within the more
robust bands to top >2"/hr Wednesday night in these areas.


The probability of significant ice >0.25" across the CONUS is less
than 10 percent.


Mullinax








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FOUS11 KWBC 090741
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
241 AM EST Mon Dec 9 2024

Valid 12Z Mon Dec 09 2024 - 12Z Thu Dec 12 2024

...CO Rockies/Sangre de Cristos...
Day 1...

Height falls moving out of the Great Basin this morning will
continue southeastward behind a cold front that has pushed into the
central Rockies. High pressure over the northern Rockies and
nosing down the High Plains will promote some modest upslope flow
into Colorado's Front Range during the day today. This upslope
component will extend as far south as the Sangre de Cristos in
northern New Mexico by tonight. Periods of snow across the region
are expected as the system moves quickly through the region and
heights build in by Tuesday. WPC probabilities are high (>70%) for
at least 4 inches of snow around the Palmer Divide southward to
Raton Pass. The highest and more remote elevations could see
localized amounts around 8-10" by the time the snow comes to an end
Tuesday morning. Lighter amounts are expected through the I-25
corridor but could be heavier around Raton Pass.

...Northeast...
Days 1-2...

Shortwave moving through the Ohio Valley this morning will help
bring in another surge of moisture coincident with an advancing
warm front. Sub-freezing air mass largely resides north of I-90,
along with some higher elevations through the Catskills, this
morning. As the warm front approaches I-80 this afternoon, an area
of low pressure will form over southeastern New England, helping to
maintain some northerly flow over northern New England. However,
with broad SW to S flow between 850-700mb, warm nose will help
support areas of freezing rain over eastern NYS into New England
today, then into Maine D2. Around a tenth of an inch of icing is
expected D1-2, but WPC probabilities for at least 0.25" are 10-20%
over western Maine (into the Kennebec Valley). Snowfall may be
limited with time due to the marginal thermal profile and lack of
cold enough temperatures in the production zone for dendrites as
the warm front attempts to push through. A few inches will be
possible toward precip onset, focusing over the Green/White
Mountains into Maine where WPC probabilities for at least 4 inches
are 40-80%.


Day 3...

By D3 (Wed-early Thu), upper pattern becomes much more amplified
upstream as a vigorous system lifts northeastward out of the
Southeast. Digging upper low into MN will raise heights over the
East Coast with milder temperatures above freezing ahead of the
front. On the backside of the front, colder air will filter in atop
a lagging precip shield, supporting a rain-to-snow transition
initially over the eastern TN Valley and central/southern
Appalachians first (Wed afternoon), then eventually into NYS Wed
evening and overnight into early Thu. WPC probabilities for at
least 4 inches of snow through 12Z Thu are low to moderate
(10-40/40-70%) over the central Appalachians (eastern WV) and into
parts of central NYS into the Adirondacks which will be quicker to
change over from rain to snow. Additionally, as the low deepens
quite smartly into Canada, cyclonic flow will wrap across Lakes
Erie/Ontario as 850mb temps crash to -10 to -15C on westerly flow.
This will support single banded lake snows by early Thursday,
continuing beyond this forecast period.


...Upper Great Lakes...
Days 2-3...

Upstream anticyclonic wave breaking over northwestern Canada will
support a digging and deep upper low moving into the Great Lakes on
Wednesday. An arctic front will switch winds to northwesterly as
much colder air is dragged into the region (850mb temps -15 to
-25C) which will kick-start the lake-effect snow (LES) machine into
high gear toward the end of D3 and continuing beyond this forecast
period. WPC probabilities of at least 4 inches of snow are high

70%) over much of the northern shore of the Michigan U.P. and

along the western coast of Michigan's Mitten. For at least 8
inches of snow, the highest probabilities lie over northwestern
Lower Michigan. Snowfall rates of 1-2"/hr are likely in more
intense bands.


Fracasso


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FOUS11 KWBC 091955
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
255 PM EST Mon Dec 9 2024

Valid 12Z Mon Dec 09 2024 - 12Z Thu Dec 12 2024

...CO Rockies/Sangre de Cristos...
Day 1...

The upper level disturbance responsible for the ongoing snowfall
throughout the region will linger through this afternoon and into
the evening hours. High pressure to the north aiding in modest
NErly upslope flow at low levels will also help to generate low-
level ascent into parts of the Front Range and on south along the
Sangre De Cristo. Latest Doppler Radar showed a potent snow band
diving south along an area of strong 700mb FGEN that is producing
1-3" of snowfall south of Castle Rock. This band has caused
hazardous travel conditions along I-25 as it tracks south through
central Colorado. This band will work through the Pueblo area soon
and then head for the Raton Mesa this evening. WPC probabilities
are high (>70%) for at least 4 inches of snow around the Palmer
Divide southward to Raton Pass. The highest and more remote
elevations could see localized amounts around 8-10" by the time the
snow concludes Tuesday morning. Lighter amounts are expected
through the I-25 corridor but could be heavier around Raton Pass.
The WSSI does depict Minor Impacts along I-25 from Pueblo on south to
Raton, NM, suggesting winter driving conditions are expected and
caution should be exercised while driving.


...Northeast...
Days 1-2...

A shortwave trough approaching the Mid-Atlantic is working in
tandem with a warm front to advect Gulf of Mexico moisture into the
Northeast. These atmospheric features are providing sources of
lift aloft to create a slug of precipitation that will fall in the
form of a wintry mix over the Northeast. In terms of snowfall,
sub-freezing temperatures through the depth of the atmospheric
columns will hold on the longest from the White mountains on north
and east through Maine this evening. These areas will ultimately
succumb to the burgeoning >0C warm nose at low-levels and
transition to an icy wintry mix later tonight. With high pressure
wedged between two areas of low pressure (one in Ontario and
another near the Massachusetts Capes) early Tuesday morning, cold
air damming (CAD) will remain entrenched over Maine and allow for a
combination of wintry mix and freezing drizzle to persist through
Tuesday night. WPC probabilities show moderate-to-high chances
(10-30%) for ice accumulations >0.1" in parts of the Adirondacks,
Green, and White Mountains. Most areas in northern New England
could see anywhere from a glaze to as much as 0.1" of ice through
Tuesday afternoon. In terms of snowfall, only the peaks of the
White Mountains sport at least low chances (10-30%) for >4" of
snowfall while minor amounts are possible in lower Maine.


...Central Appalachians...
Days 2-3...

As a sharp 500mb trough takes on a negative tilt on Wednesday,
strong divergence and height falls will result in falling snow
levels Wednesday afternoon. Periods of rain will transition over to
snow in the central Appalachians from the Laurel Highlands on south
to the central Appalachians of West Virginia at the same time low
level winds shifting out of the northwest support upslope
enhancement. Snow will be heaviest through early Thursday morning
at elevations at/above 2,000ft before tapering off later in the day
Thursday. WPC probabilities show moderate-to-high chances (40-70%)
for snowfall totals >4" from the Laurel Highlands and western
Garrett County, MD on south into east-central WV. Elevations above
3,000ft are on the higher side of those listed >4" probabilities
with low chances (10-30%) for localized amounts >6" through
Thursday morning.


...Northern Plains...
Day 2...

The leading edge of the encroaching Arctic air-mass will track into
the Northern Plains on Tuesday. Surface-based warming out in front
of the Arctic front and strong CAA aloft is allowing for lapse
rates in the 500-700mb layer to be >7.5C/km in some cases. There
will also be just enough low-level moisture for snow squalls to
develop. These squalls could form as far north and west as eastern
Montana and western North Dakota Tuesday morning then race south
and west into South Dakota Tuesday afternoon and the Missouri
Valley by Tuesday evening. Squalls could even continue to track
into parts of Iowa, northern Kansas, and northern Missouri Tuesday
night. Snow amounts will be light, but these squalls can lead to
rapid reductions and visibility. Blowing and drifting snow are
possible, along with accumulating snow on roadways where road
temperatures drop below freezing.


...Great Lakes...
Days 2-3...

The jet stream pattern over North America becomes highly amplified
as an expansive ridge along the west coast of North America helps
direct a highly anomalous upper level trough into the Upper
Midwest Wednesday evening. NAEFS shows 500mb heights over Wisconsin
that are below the 1st climatological percentile while the mean
250-500mb trough axis is negatively titled over the Great Lakes
Wednesday night. An arctic front linked to this potent upper
trough will back winds to northwesterly as bitter cold air advances
into the region (850mb temps -15 to -25C). This will reinvigorate the lake-effect snow (LES) machine Wednesday evening and into the
second half of the work-week. WPC probabilities of at least 12
inches of snow are high (>70%) northwest Michigan, from Erie
County, PA on northeast along I-90 into the southern Buffalo metro
area, and in the Tug Hill. Snowfall rates of 1-2"/hr (surpassing
3"/hr at times) are likely in the more intense bands. The WSSI-P
is showing moderate chance probabilities (40-60%) for Moderate
Impacts (disruptions to daily life; closures, hazardous driving
conditions, some detrimental impacts to infrastructure) between
Erie and the southern Buffalo suburbs on Thursday.


Mullinax





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FOUS11 KWBC 100759
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
259 AM EST Tue Dec 10 2024

Valid 12Z Tue Dec 10 2024 - 12Z Fri Dec 13 2024

...Northeast...
Day 1...

Broad SW flow in the mid-levels to the south/southeast of a
shortwave nearing James Bay will override a nose of high pressure
from Atlantic Canada, maintaining sub-freezing low-levels across
much of central/northern New England this morning. Wavy frontal
boundary will try to lift northward into the colder air, but will
be slow to succeed until tomorrow (Wed). As precipitation moves
into the region overnight tonight into Wednesday morning, freezing
rain will be likely over much of central/northern New England,
especially in areas with a little elevation. On Wednesday, southerly/southeasterly flow will eventually win out at the low
levels, turning all areas over to plain rain. Ice accumulations of
a tenth or two of an inch are likely over parts of the Green and
White Mountains and into western Maine where the cold air may hold
on the longest. WPC probabilities for at least a tenth of an inch
of ice are >50%. Though the probabilities of at least 0.25" are
low, this may be underdone as the models are sometimes too quick to
have the cold air retreat.


...Central Appalachians...
Day 2...

The southern part of the system affecting the Northeast tonight
into Wednesday will be the evolution of an increasingly negatively-
tilted and deep upper trough over the eastern 1/3 of the CONUS.
Frontal boundary will clear past the Appalachians tonight, allowing
colder air to push in behind it and change rain to snow over the
Cumberland Plateau and points eastward. Upslope enhancement into
the central Appalachians will promote more modest snow totals, and
WPC probabilities for at least 4 inches of snow are >30% over
eastern WV northward through the western MD Panhandle (Garrett Co)
into the Laurel Highlands in PA.


...Northern Plains...
Day 1...

The leading edge of the encroaching Arctic air-mass will track
into the Northern Plains today. Surface-based warming out in front
of the Arctic front and strong CAA aloft will allow for lapse rates
in the 500-700mb layer to be >7.5C/km in some cases, with just
enough low-level moisture for some snow squalls to develop. These
squalls could form as far north and west as eastern Montana and
western North Dakota this morning then race southward and eastward
into South Dakota this afternoon and the Missouri Valley by this
evening. Squalls could even continue to track into parts of Iowa,
northern Kansas, and northern Missouri tonight as the snow squall
parameter exceeds 1 in much of the CAM and even global guidance.
Though snow amounts will be light, squalls can lead to rapid
reductions in visibility with accumulating snow on roadways where
road temperatures drop below freezing and create icy hazards.


...Great Lakes/Northeast...
Days 1-3...

Upper pattern will become increasingly amplified over the East as
ridging into the West weakens but builds quite smartly over the
northern Atlantic. This will help strengthen the jet to ~170kts out
of the southwest across the TN/OH Valley overnight tonight as
arctic air moves in from the northwest. Lake effect snow will pick
up in earnest starting this evening and overnight across the U.P.
of Michigan and then into the western side of Lower Michigan as
height falls rush through (500mb heights below the 1st percentile
per the CFSR) as the trough axis becomes quite negatively-titled
into the Southeast/Mid-Atlantic. 850mb temperatures will plunge to
-15C to -25C tomorrow with plenty of Lake-to-850 DeltaT. Snow will
continue through the period as winds slowly back from NW to WNW or
W as the upper center passes by. Into the Northeast, once the
front clears the area and temperatures fall below freezing area-
wide, snow will increase off Lakes Erie/Ontario, the latter of
which will support a strong single band into the Tug Hill Plateau
with lighter snow pretty far inland. Snow off Lake Erie may also
be a single banded aimed just into the BUF southtowns. Snow will
continue across all the Great Lakes through the end of this period
(12Z Fri) but will continue beyond then.

Through 12Z Fri, WPC probabilities of at least 12 inches of snow are
high (>70%) across the favored lake belts on NW flow over the U.P.
and northwest Michigan; between Erie, PA and Buffalo, NY (esp the
southtowns but perhaps into the southern Buffalo area); and in the
Tug Hill. Snowfall rates of 1-2"/hr (surpassing 3"/hr at times) are
likely in the more intense bands.


...WA/OR Cascades...
Day 3...

Incoming shortwave or upper low will move into NorCal/Southwestern
OR Thursday, spreading some precipitation into the Cascades
southward into the Sierra. Snow levels will be around 3000ft to the
north and 5000ft to the south with QPF totals generally up to
0.50" or so. WPC probabilities for at least 6 inches of snow are
highest in the WA/OR Cascades.

Fracasso



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FOUS11 KWBC 102018
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
318 PM EST Tue Dec 10 2024

Valid 00Z Wed Dec 11 2024 - 00Z Sat Dec 14 2024

...Northeast...
Day 1...

Broad southwest flow ahead of an upper low dropping south through
Manitoba and a southern stream shortwave moving across the central
and southern Plains will override a ridge of high pressure from
Atlantic Canada, keeping sub-freezing temperatures in place at the
surface across much of central and northern New England. A wavy
frontal boundary will attempt to lift northward into the cold air,
but will be slow to make significant progress until tomorrow
(Wednesday). As precipitation moves into the region tonight and
tomorrow morning, freezing rain is expected to spread across
portions of central and northern New England, especially in areas
of higher elevation. As the day progresses, southerly to
southwesterly flow will prevail, changing precipitation to rain.
Ice accumulations of 0.05-0.10 inch, with locally heavier amounts,
are expected in the mountains of northern New England, especially
from the White Mountains eastward into northern Maine. WPC
probabilities for 0.10 inch or more have come down, however
probabilities for accumulating ice (0.01 inch) remain above 70
percent across this area.

...Central Appalachians...
Days 1-2...

The southern part of the system affecting the Northeast tonight
will evolve into a deep, negatively-tilted trough over the eastern
CONUS on Wednesday. As the associated frontal boundary moves east
of the Appalachians, colder air will fill in behind it, changing
rain to snow from the Cumberland Plateau to areas north and east.
Upslope enhancement in the central Appalachians will support modest
snow totals, with WPC probabilities of greater than 30 percent for accumulations of 4 inches or more largely confined to eastern West
Virginia.

...Great Lakes/Northeast...
Days 1-3...

Driven by falling 500mb heights and a negatively-tilted trough,
lake effect snow will develop overnight and intensify on Wednesday
across the Upper Peninsula of Michigan and into western portions
of Lower Michigan. 850mb temperatures will drop to -15C and -25C,
supporting significant lake-to-850 DeltaTs. Snow will continue
through Thursday as winds gradually turn from the northwest to
west-northwest, as the upper low dropping south through central
Canada pivots and moves east of the Great Lakes.

In the Northeast, once the front clears the region and temperatures
drop below freezing, snow will increase downwind of lakes Erie and
Ontario. Models continue to advertise intense, single-bands
targeting both the Tug Hill and the Buffalo Southtowns beginning
late Wednesday and continuing through Thursday. Some snow is
expected to continue into early Friday, but wane by late in the day
as a surface ridge builds over the region.

Through 00Z Saturday, WPC probabilities for at least 12 inches of
snow are greater than 70 percent across the favored lake-effect
snow belts of Upper Michigan, northwestern Lower Michigan, the
I-90 corridor from Erie to the Buffalo Southtowns, and the Tug Hill
Plateau. For some of these areas, especially those in New York,
WPC probabilities indicate that localized heavier amounts of 2 feet
or more can be expected.

...Cascades/Sierra Nevada...
Days 2/3...

A mid-to-upper level shortwave/compact low will drop southeast
from the eastern Pacific and move onshore on Thursday. This will
spread precipitation from the Cascades south into the Sierra. Snow
levels will be around 3000ft to the north and 5000ft to the south,
with some decreases with the passage of the shortwave. Some
additional decreases are expected Thursday night before rebounding
late in the period as a ridge begins to build and precipitation
returns ahead of a low/trough amplifying over the eastern Pacific.
Through 00Z Saturday, WPC probabilities for snow totals of 8
inches or more are highest over the Oregon Cascades into the
Shasta-Cascade region and the northern Sierra Nevada.

Pereira/Fracasso

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FOUS11 KWBC 110710
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
210 AM EST Wed Dec 11 2024

Valid 12Z Wed Dec 11 2024 - 12Z Sat Dec 14 2024


...Central Appalachians...
Day 1...

The southern part of the system affecting the Northeast tonight
will evolve into a deep, negatively-tilted trough over the eastern
CONUS on Wednesday. As the associated frontal boundary moves east
of the Appalachians, colder air will fill in behind it, changing
rain to snow from the Cumberland Plateau to areas north and east.
Upslope enhancement in the central Appalachians will support modest
snow totals, with WPC probabilities of greater than 30 percent for accumulations of 4 inches or more largely confined to eastern West
Virginia but also northward into the Laurel Highlands.


...Great Lakes/Northeast...
Days 1-3...

Driven by falling 500mb heights and a negatively-tilted trough,
lake effect snow will intensify today across the Upper Peninsula of
Michigan and into western portions of Lower Michigan. 850mb
temperatures will drop to -15C and -25C (below the 5th percentile),
supporting significant lake-to-850 DeltaTs. Snow will continue
through Thursday while winds gradually turn from the northwest to
west- northwest, as the upper low dropping south through central
Canada pivots and moves east of the Great Lakes.

In the Northeast, once the front clears the region and
temperatures drop below freezing, snow will increase downwind of
lakes Erie and Ontario. Models continue to advertise intense,
single-bands targeting both the Tug Hill and the Buffalo Southtowns
beginning later today and continuing through Thursday. Snow will
gradually wind down from west to east across the Great Lakes
starting late Thu/early Fri before finally ending early Saturday
over NY as a surface ridge builds over the region.

For the event (next 72 hours), WPC probabilities for at least 12
inches of snow are greater than 70 percent across the favored lake-
effect snow belts of Upper Michigan, northwestern Lower Michigan,
the I-90 corridor from Erie to the Buffalo Southtowns, and the Tug
Hill Plateau. For some of these areas, especially those in New
York, WPC probabilities indicate that localized heavier amounts of
2 feet or more can be expected. Hazardous conditions can be
expected in the more intense bands where snowfall rates may exceed
2 inches per hour on Thursday per the 00Z HREF.

...Cascades/Sierra Nevada...
Days 1-3...

After a recent quiet period, the pattern will become more active
in the West over the next few days. A shortwave/compact upper low
will move ashore on Thursday, with some precipitation out ahead of
it today/tonight over the OR Cascades and the northern CA ranges.
Snow levels will be around 3000ft to the north and 5000ft to the
south, decreasing with the passage of the shortwave. Some
additional decreases are expected Thursday night before rebounding
late in the period as a ridge begins to build and precipitation
returns ahead of a low/trough amplifying over the eastern Pacific.
That system will have a little more moisture and amplitude,
focusing the precipitation northward into the WA Cascades by
Friday. Greatest impacts will be at the mountain passes throughout
the region from the WA Cascades southward into the norther Sierra.
WPC probabilities for 3-day snow totals of 12 inches or more are
highest over the Oregon Cascades into the Shasta-Siskiyous/Trinity
region and the northern Sierra Nevada.

...Corn Belt...
Day 3...

Upper low exiting the Rockies on Friday will start to tap limited
moisture out of the Gulf, eventually promoting an area of WAA-
driven precipitation across the Corn Belt. Cold air mass in place
beneath a warming ~850mb layer will favor an area of mixed
precipitation (sleet/freezing rain) just south of some light snow
on the northern side where the column remains below freezing.
Amounts may be light, but any freezing rain could be hazardous. WPC probabilities for at least 0.1" of ice accumulation are 10-30%
over IA through 12Z Saturday.


The probability of significant icing greater than 0.25 inches is
less than 10 percent across the CONUS this period.

Fracasso/Pereira




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FOUS11 KWBC 111959
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
259 PM EST Wed Dec 11 2024

Valid 00Z Thu Dec 12 2024 - 00Z Sun Dec 15 2024


...Great Lakes/Northeast/Central Appalachians...
Days 1-2...

Deep negatively-tilted trough as low as 510 dam (below the 1st
percentile) diving over the Great Lakes along with very cold
temperatures will continue to produce heavy lake-effect snow over
the Upper Peninsula of Michigan and into western portions of Lower
Michigan through Thursday. The eastern lakes (Erie and Ontario)
will also develop significant lake-effect snowbands tonight through
Thursday as the trough nears the region and a developing surface
low along a strong cold front swings across northern New England.
Models highlight intense, single snowbands targeting the Tug Hill
and Buffalo Southtowns. Pronounced lake-850 DeltaTs are anticipated
from -15C to -25C 850mb temperatures (below the 5th percentile
across lakes Erie and Michigan), with the lakes remaining ice-free
and surface water temperatures between 5C and 9C. Snow will
continue through Thursday while winds gradually turn from the
northwest to the west-northwest as the upper low swings eastward
over the Great Lakes and into southeast Canada. Snow will linger a
bit longer downwind of the eastern Lakes into early Friday until a
potent surface high moves directly overhead.

Total lake-effect snowfall probabilities from WPC over the next 48
hours are highest over the favored snowbelts of Upper Michigan,
northwestern Lower Michigan, the I-90 corridor from Erie to the
Buffalo Southtowns, and the Tug Hill Plateau. Chances for at least
12 inches of snow are high (>80%) in these areas. For some regions,
especially eastern New York and into Erie, PA, the potential exists
for over 2 feet of snow and snowfall rates over 2 inches per hour.
This would create hazardous to potentially impossible driving
conditions.

For the Northeast and central Appalachians, cold air infiltrating
behind an intense moisture plume riding up the East Coast tonight
will allow for a brief changeover to snow throughout interior
locations. The heaviest snow (up to 4 inches) is expected early in
the D1 period across the central Appalachians due to favorable
upslope flow into Thursday, with a few inches possible in the
higher terrain of northern New England.

...Cascades/Sierra Nevada...
Days 1-3...

After a recent quiet period, the pattern will become more active
in the West over the next few days. A shortwave/compact upper low
will move ashore on Thursday, with some precipitation out ahead of
it tonight over the OR Cascades and the northern CA ranges. Snow
levels will be around 3000ft to the north and 5000ft to the south,
decreasing with the passage of the shortwave. Some additional
decreases are expected Thursday night before rebounding by Friday
night as a ridge begins to build and precipitation returns ahead
of a low/trough amplifying over the eastern Pacific. That system
will have a little more moisture and amplitude, focusing a broader
area of precipitation northward from the northern CA ranges into
the WA Cascades by Friday, with the heaviest precipitation expected
over northern CA Fri-Sat associated with an Atmospheric River. IVT
values are expected to peak around 700 kg/m*s over northern CA,
which will drive snow levels above 4500ft (over 5500ft in the
Sierra). Greatest impacts will be at the mountain passes throughout
the region from the WA Cascades southward into the norther Sierra.
WPC probabilities for 3-day snow totals of 12 inches or more are
highest over the Oregon Cascades into the Shasta-Siskiyous/Trinity
region and the northern Sierra Nevada, with the northern CA
potential seeing the greatest impacts from high elevation heavy wet
snowfall.

...Corn Belt...
Day 3...

Upper low exiting the Rockies on Friday will start to tap limited
moisture out of the Gulf, eventually promoting an area of WAA-
driven precipitation across the Corn Belt. Cold air mass in place
beneath a warming ~850mb layer will favor an area of mixed
precipitation (sleet/freezing rain) just south of some light snow
on the northern side where the column remains below freezing.
Amounts may be light, but any freezing rain could be hazardous. WPC probabilities for at least 0.1" of ice accumulation are 20-50%
over IA through Saturday night.


The probability of significant icing greater than 0.25 inches is
less than 10 percent across the CONUS this period.

Snell






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FOUS11 KWBC 120806
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
306 AM EST Thu Dec 12 2024

Valid 12Z Thu Dec 12 2024 - 12Z Sun Dec 15 2024


...Great Lakes..
Days 1-2...

The core of an anomalous upper low (-2 sigma at 500mb according to
NAEFS with a 510dm center) will begin to fill and drift eastward
across southern Canada on D1, with the primary mid-level trough
axis pivoting into New England by 00Z Saturday. This will be
replaced by brief shortwave ridging downstream of a potent trough
digging across the Central Plains. Until that happens, however, the
environment will remain favorable for another round of widespread
and heavy lake-effect snow (LES) in the W/NW snow belts.

At the start of the forecast period, 850mb temps will be generally
-15C to -20C, and continue to plunge to -18C to -23C by 00Z Friday
before slow warming begins. This CAA supplying the cold air will
move across Great Lakes water temperatures that are still warm,
generally +5 to +9C, supporting deltaT that is well above
threshold for heavy LES, and will drive lake-induced equilibrium
levels 10,000 - 15,000 ft, which will be well above the -10C
isotherm suggesting potential lightning in the heavier bands. This
will support snowfall rates of 1-2"/hr in many of the bands, with
locally 3-4"/hr possible downstream of Lakes Erie and Ontario.
Combined with the cold temperatures and gusty winds, significant
travel impacts are expected within LES.

The heaviest LES is likely D1 before a slow ease of CAA (and onset
of subtle WAA) wanes LES from west to east during D2, but not
before multiple feet of snow occur in the more persistent and
intense banding. WPC probabilities D1 are high (>70%) for more than
8 inches in the eastern U.P., near Traverse Bay, along the
Chautauqua Ridge, and into the Tug Hill Plateau, with high
probabilities continuing on D2 only in the Tug Hill.


...Pacific Coast and Interior Northwest...
Days 1-3...

Two waves of precipitation will spread onshore the West Coast and
then inland through the forecast period bringing widespread snow
to the higher elevations.

The first wave of moisture will be ongoing to start the period as a
slowly filling closed low drops into northern CA with 500mb height
anomalies falling below -1 sigma according to NAEFS. Height falls
combined with the LFQ of an accompanying upper level jet streak
will drive ascent onshore, leading to locally heavy snow, primarily
confined to the Sierra Nevada. The strongest forcing is transient
and pushes south of the area halfway through D1, but at least a
short duration of 1"/hr snow rates is likely across the Sierra,
generally above 5000 ft. During D1, WPC probabilities for more than
6 inches of snow are above 70% in the Sierra, but probabilities for
more than 12 inches are only 10-30%.

A more significant system will then approach the coast from the
Pacific Friday into Saturday, leading to a more substantial
precipitation event after only a brief break Thursday night into
Friday. This system will again be driven by a pronounced closed low
offshore, opening into a potent trough as it shifts across CA/OR
late Saturday, reaching the Great Basin by the end of the forecast
period. Once again, downstream divergence, warm/moist advection,
and robust jet energy will combine to drive ascent. However, this
second system will be more broad, reflected by 500mb height
anomalies below -1 sigma across much of the Pacific Coast, combined
with modestly coupled jet streaks to push a strong surface low into
British Columbia and a second wave across CA. This overall more
impressive system is reflected as well by GEFS and ECENS
probabilities for 500 kg/m/s IVT lifting onshore exceeding 60%.

Snow levels across the West with this second system will generally
remain around 4000-5000 ft, with a narrow corridor exceeding 6000
ft in the strongest IVT/WAA. However strong ascent into an
environment with a deep layer of lapse rates stronger than moist-
adiabatic could lower snow levels beyond forecasts, and the NBM
25th percentile may be more representative, which reaches as low as
3000-4000 ft during the heaviest precipitation, and falls even
lower as precip begins to shift eastward. This suggests the
potential for more widespread pass-level impacts, reflected by
WSSI-P above 70% for moderate impacts due primarily to snow load
and snow rate in the Sierra and Shasta/Trinity region. Lesser, but
still notable impacts, are possible as far north as the WA
Cascades.

WPC probabilities for more than 8 inches of snow on D2 are high

70%) but confined to the Shasta/Trinity region of northern CA and

the highest peak of the WA/OR Cascades. By D3, high WPC
probabilities for more than 8 inches spread across the length of
the Sierra and into the Sawtooth/Salmon River Ranges, while also
continuing near Mt. Shasta.


...Central Plains into the Upper Midwest...
Days 2-3...

A closed 500mb low emerging from the Central Rockies will track
eastward while deepening and pulling south in response to a potent
vorticity lobe rotating around its base. The trend in the global
guidance has been for this feature to be a little weaker and
farther south, and while this will likely result in more
interaction with a stationary front/modest low-level baroclinic
zone, the resultant downstream moisture advection may be somewhat
muted. Still, as this advects east into Missouri by Saturday
evening, it will combine with strengthening fgen in response to
surging WAA to drive moisture northward reflected by a narrow
channel of +1 sigma PWs on the NAEFS tables, while also helping to
create a surface low moving across the Central Plains and the Upper
Midwest by the end of the forecast period.

Downstream of this low, a sprawling high pressure centered over the Mid-Atlantic states will gradually retreat to the east. This will
allow confluent flow to merge into the Corn Belt and then Upper
Midwest, with overrunning WAA leading to a period of mixed
precipitation including sleet and freezing rain, with snow farther
north. Even where precipitation is snow, it may begin as a period
of ZR/IP due to antecedent low RH within the DGZ (no ice growth)
noted in regional soundings.

There remains considerable spread in the latitudinal gain of warm
air, but with the high retreating steadily and WAA increasing,
solutions with a farther north depiction of accumulating snow and
ice seem more reasonable, and this could result in hazardous
accretions of freezing rain, and modest snowfall as well. Current
WPC probabilities for more than 4 inches of snow are less than 10%
with this system, but the probability for at least 0.1 inches of
ice are 10-30% D2 in central IA, increasing to 50-70% D3 when
locally as much as 0.2 inches is possible.

Weiss


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FOUS11 KWBC 122040
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
340 PM EST Thu Dec 12 2024

Valid 00Z Fri Dec 13 2024 - 00Z Mon Dec 16 2024


...Great Lakes..
Day 1...

Surface high centered over central MN this afternoon will amplify
and shift east over southern WI tonight and then to Lake Erie
through Friday. Flow veers north from Wly and weakens as the high
approaches which will cutoff the ideal LES conditions continuing
over the Great Lakes. Banding in MI and off Lake Erie diminishes
quickly after midnight tonight with Day 1 PWPF for >6" 40% or less
in the typical Wly flow snow belts.
However, potent single band LES persists through Friday morning
for the Tug Hill area east of Lake Ontario. Day 1 PWPF for >12"
here is around 70% with max additional around 18".


...Western U.S...

An initial wave over CA/NV tonight weakens by Friday with a longer
wave approaching the PacNW coast Friday night before sweeping over
the northern Rockies through Sunday with heavy mountain snows
expected.

California...
Days 1-2...

A reinforcing trough currently over the southern OR Coast shifts
south behind the main trough axis currently extending down the
Sierra Nevada. This brings additional heavy snow to the northern
Sierra Nevada 00Z-06Z tonight with snow levels around 4000ft. Snow
rates above the snow level will top 1"/hr until dropping off
quickly after 06Z per the 12Z HREF. Day 1 PWPF for >6" over the northern/central Sierra are 50-80%.

At atmospheric river ahead of the next system arrives into the far
northern CA coast midday Friday with snow levels quickly rising
over 5000ft on the Trinity Alps/Klamath/Siskiyou. Heights begin to
fall with the approaching trough Friday night with snow levels
decreasing to 4000ft by 12Z Saturday under the trough axis. A
potent plume of moisture surges over the length of the Sierra
Nevada on Saturday which produces a multi-hour bout of heavy snow.
Ridging then builds in a quickly ends heavy snow by Saturday
evening. Day 2 PWPF for >12" is 50-80% for the length of the Sierra
Crest along with the CA Cascades.

Pacific Northwest through Northern Rockies...
Days 2-3...

Moisture streams over the PacNW coast later Friday with moderate to
locally heavy snow above the 4000ft snow levels in the WA Cascades
and 5000ft in the OR Cascades. These snow levels drop about 1000ft
under the trough axis on Saturday. Day 2 PWPF for >8" are 40-80%
for the higher Cascades.

The moisture surge reaches the northern Rockies Friday night with
heavy snow focusing mainly over central ID terrain through Saturday
with snow levels 4000-5000ft. Day 2.5 PWPF for >8" are 50-90% over
the Sawtooth and southern Salmon River Mtns as well as the Blues in
OR.

As the trough crosses the northern Rockies Saturday night into
Sunday, the terrain from Yellowstone to the Wasatch sees heavy
snow with the Day 3 PWPF for >8" is 50-80% with snow levels
dropping from 6000ft to 5000ft through the event.



...Central Plains into the Upper Midwest...
Day 2...

A trough currently over the Sierra Nevada amplifies as it shifts
east of the CO Rockies Friday night, closing over KS. Gulf-sourced
moisture will stream over cold ground over eastern Neb/IA to
produce a wintry mix late Friday into Saturday before shifting ENE
to MI through Sunday. The combination of warm air advection over
areas that had been under a cold 1040mb high makes for a notable
freezing rain threat with a modest snow/sleet threat. Day 2 PWPF
for >0.1" ice are above 20% from northeast Neb across IA into
southern MN, as well as the Driftless region of southwest WI with a
large area over 60% in central and eastern IA.


...Central Appalachians...
Day 3...

Cold air in the wake of the aforementioned 1040mb high lingers over
the central Appalachians on Sunday with freezing rain threats. Day
3 PWPF for >0.1" ice are 20-30% in the Potomac Highlands of MD/WV
into the Laurels of southwest PA.


Jackson





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FOUS11 KWBC 130813
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
313 AM EST Fri Dec 13 2024

Valid 12Z Fri Dec 13 2024 - 12Z Mon Dec 16 2024


...Great Lakes..
Day 1...

Residual heavy lake-effect snow (LES) will persist through the
first half of Friday, especially east of Lake Ontario into the Tug
Hill Plateau, before increasing SW flow downstream of a shortwave
moving through the middle of the country causes WAA and shuts off
LES by the end of D1. Before that occurs, the environment remains
favorable for 2+"/hr snow rates, primarily downstream of Lake
Ontario, before intensity wanes and LES ends later in the day.
Despite a generally short duration of continuing heavy LES, WPC
probabilities for more than 6 inches are high (>70%) downstream of
Lake Ontario, with locally up to 12 inches possible.


...Western U.S...

A closed low over the eastern Pacific will amplify and shed
periodic vorticity lobes towards the Pacific Coast of the United
States and Canada. Each of these will combine with increased
moisture to cause widespread winter weather across the region.

California...
Days 1-2...

One of these shortwaves will intensify into a pronounced trough,
taking on a negative tilt as it shifts into central CA Saturday
aftn /early D2/. This will act in tandem with modestly coupled jet
streaks to drive ascent, while impressive moist advection surges
IVT to 500-750 kg/m/s according to both GEFS and ECENS
probabilities. Snow levels will rise to around 5000 ft within the
core of this IVT, highest in the Trinity/Shasta/Siskiyou region,
but still support heavy snow in the higher terrain, with
accumulations aided by nearly ideal upslope flow. Although the
heaviest snowfall, which will likely feature rates above 1"/hr will
occur within the warmer snow levels/higher IVT, even as snow levels
fall behind the primary trough axis moderate to heavy snow will
persist through D2 before waning. Additionally, heavy snow through
synoptic and impressive upslope flow will extend along the length
of the Sierra, bringing substantial impacts to many of the Crest
Passes. WPC probabilities for more than 12 inches of snow is
extremely high (>90%) in the Shasta/Trinity region and the northern
Sierra D1, continuing above 70% D2 while extending down the length
of the Sierra. Local snowfall maxima of more than 4 feet are likely
in the highest terrain.


Pacific Northwest through Northern Rockies...
Days 1-2...

Farther north, a stronger closed mid-level low will push into
British Columbia as it remains embedded in the downstream southerly
flow ahead of the larger scale trough axis. This will drive
persistent moist advection into the Pacific Northwest D1 into D2,
with this moisture then merging with the aforementioned shortwave
ejecting from CA to drive widespread heavy precipitation into the
Northern and Central Rockies. Snow levels will fluctuate through
the period, but generally remain around 4000-5000 ft. However,
strong ascent and ample moisture could result in locally lower snow
levels through cold-air dragging on intense snowfall rates,
especially in regions of intense upslope ascent or modest elevated
instability.

WPC probabilities for more than 6 inches of snow are high (>80%)
along the WA/OR Cascades, the Olympics, and into the Salmon
River/Sawtooth region on D1, with the highest probabilities
focusing across parts of ID and into the Tetons D2, while
remaining, albeit more modestly, in the Cascades. By D3 the highest
WPC probabilities for more than 6 inches of snowfall drop
considerably to 30-50%, focused almost exclusively in the Tetons.
Storm total snowfall of 20-30 inches likely in the higher terrain
of many of these ranges. With snow levels ranging between around
4000-5000 ft, some of the higher passes could experience
considerable impacts as well.


...Central Plains into the Upper Midwest...
Days 1-2...

A trough ejecting from the Central Rockies Friday night will
amplify into a closed low over the Central Plains as it digs E/SE
through Saturday, and then lifts northeast towards the Great Lakes
on Sunday. Height falls downstream of this deepening trough will
combine with weak but coupled jet streaks aloft and a residual
low-level baroclinic gradient to drive surface cyclogenesis in the
lee of the Rockies Friday night. As this low moves progressively to
the east, it will draw increased moisture northward from the Gulf
of Mexico, channeling an axis of PW anomalies as high as +2 sigma
into MO/IA Saturday. This moisture has steadily shown an increased
trend with recent model runs, likely in response to a subtly deeper
upper low.

As this feature moves east and deepens, it will interact with a
retreating but sprawling surface high (max pressure around
1050mb!). This will leave an environment that is cold enough for
wintry precipitation, but as the WAA intensifies, the high will
lose its favorable position for cold advection leading to a column
that will become overwhelmed by the WAA and warm with time.
Additionally, it will take some time for the DGZ to saturate as the
antecedent air is quite dry, so this setup continues to look more
favorable for IP (and mostly ZR), with just some snow on the
northern side. However, the stronger low could also result in some
more intense deformation to the north of the surface low, leading
to at least some modest snowfall accumulations in MN/WI.

Despite that, the primary hazard appears to be freezing rain, and
WPC probabilities have increased for significant accretion, with
locally damaging icing now possible as reflected by max
probabilities reaching around 30% for > 0.25" of ice, highest
across central IA.


...Central Appalachians...
Day 3...

The same high pressure retreating eastward from the Upper Midwest
(above) will push into New England and then favorable wedge down
the coast in a Cold Air Damming (CAD) setup east of the
Appalachians. As moisture from a low pressure moving into the Great
Lakes shifts eastward, it will encounter cold air, especially in
the higher elevations, supportive of wintry precipitation. However,
the robust WAA driving the expanding precipitation shield is
likely to overrun the cold air, leading to a corridor of
significant freezing rain, with only light snow accompanying. WPC
probabilities for more than 0.1" of freezing rain have increased to
as high as 50-70% across the higher terrain from eastern WV,
through the western Panhandle of MD, and into the Laurel Highlands
of PA.

Weiss



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FOUS11 KWBC 132106
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
406 PM EST Fri Dec 13 2024

Valid 00Z Sat Dec 14 2024 - 00Z Tue Dec 17 2024


...Lake Ontario...
Day 1...

Lingering single band in westerly flow over Lake Ontario will
persist into this evening for the Tug Hill area before diminishing
around midnight. A few more inches are likely after 00Z.


...Western U.S...

A powerful shortwave trough rounding a low centered west of WA
will shift over the CA coast early Saturday before tracking over
the northern Rockies through Sunday. The next plume of moisture
arrives Sunday night into the PacNW Coast ahead of a weakening low
off Vancouver Island. A less potent onshore flow then expands over
the Northwest through Monday.

California...
Day 1...

A moderately strong atmospheric river will continue to stream in
over far northern California tonight before a potent and digging
vort lobe crosses the SF Bay area around 15Z Saturday. This sharp
trough will provide an intense focus for lift and orthogonal flow
to the Sierra Nevada on Saturday with a quick end Saturday evening
behind the trough axis. Snow levels linger around 5000ft over the
CA Cascades and Sierra Nevada through tonight before rising perhaps
500ft just ahead of the wave Saturday. Heavy snow through synoptic
and upslope flow will extend along the length of the Sierra with
high rates persisting longest over the northern half. Per 12Z HREF,
hourly rates exceeding 1"/hr resume around 08Z for the northern
Sierra where they persist until about 22Z with the southern Sierra
seeing heavy rates from about 18Z to 00Z. Max rates exceed 2"/hr
for a few hours as the max precip works its way south down the High
Sierra 14Z to 22Z. The additional snowfall maxima for Day 1 alone
is around 36" in the highest western slopes of the northern and
central Sierra.


Pacific Northwest through Northern Rockies...
Days 1-2...

Farther north, the closed mid-level low pushes ENE over Vancouver
Island late tonight. Persistent moist advection into the Pacific
Northwest occurs through Saturday night as an additional
reinforcing shortwave trough swings through on Sunday driving
widespread heavy precipitation into the Northern Rockies through
Sunday. Snow levels will fluctuate through the period, but
generally remain around 4000-5000 ft. However, strong ascent and
convection could result in locally lower snow levels during heavier
activity. This configuration of back-to-back waves consolidating
over the NW focuses the inland heavy precip on central ID terrain.

Day 1.5 WPC probabilities for >8" are moderately high (50-80%)
along the WA/OR Cascades and the Olympics and high (>80%) over the ,
Salmon River/Sawtooths in ID along with the Wallowa Mtns in
northeast OR. This then expands to western WY down to the Wasatch
where Day 3 snow probs for >8" are 50-80%, highest in the Tetons.
Expect considerable impacts in the passes here.

...Pacific Northwest...
Day 3...

The next wave has an initial burst along the Cascades early Monday
with snow levels around 2500ft in WA and 3500ft in OR. However, the
weakening low and diminishing moisture influx means more moderate
rates through the rest of Monday as snow levels slowly rise. Day 3
snow probs for >8" are 30-60% for the Cascades above those snow
levels.



...Central Plains into the Upper Midwest...
Days 1-2...

A trough currently ejecting from the Central Rockies will amplify
into a deep low over KS tonight before shifting over northern IN
through Sunday. Height falls downstream of this developing low
will combine with weak but coupled jet streaks aloft and a residual
low-level baroclinic gradient to drive surface cyclogenesis over KS
by this evening. As this low progresses ENE, increased Gulf
moisture will be drawn north and around the low and over the I-80
corridor around IA which has been under and Arctic airmass the past
couple days, resulting in a wintry mix. Despite the fairly strong
warm air advection, the plain rain line looks to only modestly work
northward through Saturday night as precip continues over the
Upper Midwest. It will take some time for the DGZ to saturate as
the antecedent air is quite dry, so this setup is better for
freezing drizzle turning into freezing rain/sleet, and snow on the
northern end. Day 1 ice probs for >0.25" are 20-40% over both
western and eastern IA with coverage of >0.1" ice extending from
eastern Neb to just beyond the northern and eastern IA borders.


...Central Appalachians...
Days 2/3...

The same high pressure retreating eastward from the Upper Midwest
will drift over New England Saturday through Sunday, allowing a
strong Cold Air Damming (CAD) wedge to setup east of the
Appalachians over the Mid-Atlantic. Moisture from low pressure
moving into the Great Lakes shifts eastward over the central Apps
Saturday night. Air will initially be cold enough for snow,
especially in the higher elevations, before the robust WAA brings a
wintry mix that lingers over terrain and preferred higher central
App valleys into or through Sunday night. WPC probabilities for

0.1" of freezing rain are highest 12Z Sun-12Z Mon (Day 2.5) with

70-80% over the Laurels of PA down into the MD Panhandle with >20%
probs extending from north-central PA through southern WV.


...Northern Plains...
Days 2/3...

The potent trough axis crossing SF Bay early Saturday lifts ENE
over the northern Rockies early Sunday with further development
over ND on Sunday. Overrunning flow ahead of the wave brings a
light freezing rain threat to northeastern MT Saturday night into
Sunday morning with the Day 2 ice probs for >0.1" 20-60%. Snow
bands then develop north of the low with Day 3 snow probs for >4"
around 20% for the north-central border of ND.


Jackson


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FOUS11 KWBC 140817
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
317 AM EST Sat Dec 14 2024

Valid 12Z Sat Dec 14 2024 - 12Z Tue Dec 17 2024


...Western U.S...

An active period will bring widespread winter weather to much of
the West through early next week.

A large trough just offshore the Pacific Coast to start the period
will shed two distinct compact vort maxima/shortwaves onshore on
D1. One of these will pivot northward into British Columbia, while
the second one ejects into central CA. Although the northern
impulse is a bit deeper relative to the CFSR climatology (500mb
heights below the bottom 2.5 percentile according to NAEFS), the
greater moisture/IVT will pivot into CA associated with the
southern system as the northern moisture channel lifts into Canada. Additionally, there will be a modestly coupled jet streak
downstream of the primary trough axis and in the vicinity of this
southern impulse, helping to enhance ascent to spread more
widespread moisture northeast. On D1, this will cause widespread
heavy snow from the Sierra and Shasta/Trinity region eastward into
the Salmon River/Sawtooth Ranges, Blue Mountains, and as far east
as the Tetons and Big Horns, WPC probabilities are high (>70%) for
above 12 inches in the CA ranges and into ID thanks to higher
moisture, strong synoptic lift, and impressive upslope flow driving
snowfall rates of 2+"/hr at times. 1-2 feet of snow is possible D1
across these areas.

More modest snowfall across the WA/OR Cascades and Olympics where
both ascent and moisture are more limited. Still, WPC probabilities
in the Cascades and Olympics are moderate to high (50-90%) for
more than 8 inches. Snow levels D1 will climb briefly to as high as
5000-6000 ft in the core of the max IVT/WAA/moisture plume, but
will fall gradually most of the day to as low as 2000 ft in the
Cascades and 3500 ft elsewhere. This will allow at least modest
accumulations below pass levels, but most of the accumulating snow
is likely during the period of higher snow levels.

On D2, the pattern evolves quickly as a narrow but amplified
longwave ridge builds over the Pacific Coast, forced between the
departing trough now moving into the northern High Plains and a
renewed closed low back over the Pacific Ocean. This will force a
respite in precipitation across the coast and into the Great Basin,
but wintry precipitation will continue into the Northern Rockies,
but in a slowly waning fashion. Snow levels will continue to fall
steadily D2, reaching below 1000 ft in the Northern Rockies, which
coincident with continuing (but easing) snow will result in heavy
snow accumulations across the NW WY ranges, with some light
accumulations down into most of the valleys. WPC probabilities D2
are moderate (50-70%) for an additional 6+ inches, highest in the
Tetons.

Late D2 and then more substantially into D3, the large closed low
west of WA/OR will spin two distinct vorticity maxima/shortwaves
onshore, leading to increasing waves of precipitation spilling
eastward once again. With this next impulse, snow levels will
initially be low, only around 1500 ft, with modest rising
occurring in the axis of greatest IVT. Despite IVT progged to
remain only around 250 kg/m/s, this will lift snow levels to around
3500 ft west of the Cascades in WA/OR, and then potentially as
high as 4500 ft later D3 with the second wave. With impressive
ascent increasing, and upslope flow amplifying into the terrain,
heavy snowfall will result, and WPC probabilities D3 are moderate
(50-70%) for 6+ inches from the Shasta/Trinity region northward
along the Cascades.

Moisture spilling east into the interior Northwest will encounter
some trapped cold air and modest easterly flow ahead of the
accompanying WAA/IVT, such that some light freezing rain could
result on the eastern side of the Cascades. Potentially more
impressively, the accompanying WAA and associated fgen could result
in heavier snow rates farther east which could reach the valley
floors of interior WA and OR. Confidence is low this far out, but
WPC probabilities do indicate at least a low chance for 1" of snow
across much of WA east of the Cascades, with heavier snow likely
moving back into the Salmon River/Sawtooth Ranges before the end of
the period.


...Central Plains into the Upper Midwest...
Day 1...

A mixed snow, sleet, and freezing rain event will be ongoing to
start the forecast period as a wave of low pressure moves steadily
from Kansas into the western Great Lakes. This low will be driven
by a compact but potent closed low moving over-top the surface
low, with downstream warm and moist advection expanding the
precipitation shield northeast through the region. As this
vertically stacked system moves east, it will run into some cold
and dry antecedent air as an expansive high pressure gradually
retreats towards New England. As WAA intensifies, it will push a
warm nose above 0C, while surface wet-bulb temperatures will remain
below freezing. The guidance has narrowed the corridor of
significant freezing rain, and despite what could be moderate
precip rates (supported by soundings) and a lack of dry advection
to offset latent heat release of freezing (and to prevent
substantial warming), there is high confidence in a stripe of
significant ice accretion, especially in eastern and central IA.
Here, WPC probabilities are moderate (50-70%) for an additional
0.1+ inches of ice, leading to local event totals exceeding 0.25".


...Central Appalachians...
Days 1-2...

Sprawling high pressure with impressive central pressure
approaching 1050mb will gradually pivot east through New England
during the weekend. While this feature will retreat, it will extend
down the coast east of the Appalachians as a cold-air damming
scenario, at least initially reinforced by mid-level confluence
ahead of an approaching shortwave. This impulse will lift from the
Corn Belt late Saturday to off the New England coast, with the
associated moisture and ascent producing a swath of precipitation
extending from the Southern Appalachians through southern New
England. The WAA overrunning the cold surface layer will likely
result in an axis of moderate to heavy freezing rain in the higher
elevations from southern WV through the MD Panhandle and into the
Laurel Highlands where WPC probabilities for more than 0.1 inches
of ice reach as high as 70%, and locally in excess of 0.25" is
possible near the MD Panhandle and into the southern Laurel
Highlands which is where the greatest duration of freezing rain is
expected. Surrounding this, light freezing rain could result in
accumulating ice exceeding 0.01 inches as far south as the northern
escarpment of SC, and as far north as some of the higher terrain of
Upstate NY.

While the most significant winter impacts are likely due to
freezing rain, many areas from PA through MA will experience
precipitation at least starting a snow with some light
accumulations likely in the higher elevations of the Poconos,
Catskills, and Upstate NY/western New England. At this time
however, WPC probabilities are only 50-70% for 2+ inches in these
areas. In places that receive mostly rain, a brief mix of snow and
sleet is possible during precipitation onset even as far southeast
as the I-95 corridor, but no accumulation is expected.


...Northern Plains...
Days 2-3...

Vorticity maxima leftover from a weakening shortwave will lift
northeast from the Great Basin, with secondary intensification of
this feature likely over eastern MT Sunday. The guidance has
continued to trend a bit deeper and farther south with this
secondary development, suggesting a higher potential for some
moderate to heavy precipitation from MT into ND.

As the lead shortwave weakens and pushes northeast, some leading
WAA will expand light precipitation into the northern High Plains.
Forecast soundings suggest considerable dry air to overcome, but
as this occurs it will cause some wet-bulb effects to keep temps
below freezing and an axis of light freezing rain is likely from
eastern MT into western ND Saturday night into Sunday morning.
Accumulations are likely to be modest, however, as reflected by WPC probabilities for 0.1+" of ice peaking around 30% in eastern MT on
D2. Still, any freezing rain can cause hazardous travel
conditions.

Thereafter, the secondary impulse deepens into a closed low which
should cause some enhanced deformation to the north and west of the
system. This deformation acting upon some higher theta-e air
wrapping cyclonically around the low (emerging from intensifying
290K isentropic ascent downstream) will support some banded snow
rotating through ND and then into MN Sunday night and Monday. At
this time total snowfall is still expected to be light however,
with WPC probabilities for 2+ inches of snow reaching just around
50% from eastern ND into northern MN.


Weiss


$$

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FOUS11 KWBC 142005
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
305 PM EST Sat Dec 14 2024

Valid 00Z Sun Dec 15 2024 - 00Z Wed Dec 18 2024


...Western U.S...
Days 1-3...

A trio of storm systems will make for an active stretch of wintry
weather to much of the West through early next week.

An elongated 500mb trough tracking through the Northwest will
direct the IVT responsible for the heavy snow in the Sierra Nevada
today into the northern Rockies this evening. According to NAEFS,
the IVT is ~100-200 kg/m/s in parts of the Intermountain West by
00Z this evening, which is above the 90th climatological percentile
from Southern CA to as far north and east as Wyoming. As snow
winds down in the Sierra Nevada by tonight, this moisture source
will work in tandem with broad diffluent flow at 250-500mb aloft to
support periods of snow across the northern Rockies through Sunday
afternoon. Farther west, persistent onshore flow will result in
more mountain snow in the Olympics and Cascades before high
pressure builds briefly Sunday afternoon. Through 00Z Monday, WPC
PWPF shows moderate-to-high probabilities (50-70%) for snowfall
totals >6" in the WA/OR Cascades at/above 5,000ft. The Tetons and
Big Horns have the best odds (high chances, or >70%) for snowfall
totals >8" with the tallest peaks of the Tetons likely receiving
between 1-2 feet of snow. It is worth mentioning that portions of
the Absaroka, Little Belt, and Big Snowy Mountains in Montana also
have moderate-to-high chances (50-70%) for snowfall totals >6". The
WSSI shows Moderate Impacts in parts of the Tetons and Big Horns
through Sunday afternoon.

By Sunday night, the brief reprieve in the Pacific Northwest ends
as yet another Pacific storm system approaches the region. This is
due to a shortwave trough breaking off from its parent upper low
located in the northeast Pacific. Mountains snow from the Olympics
and Cascades on south to the Siskiyou and Trinity/Shasta begin
early Monday morning, peaking around late morning in intensity, and
snowfall rates diminishing to a degree by Monday afternoon. The
upper low will then make its way towards the region Monday night,
albeit weakening on approach. Still, a healthy plume of 850-700mb
moisture will allow for more heavy mountains snow in the Cascades
through Tuesday morning before snow levels rise above 5,000ft
Tuesday afternoon. The WSSI-P shows moderate probabilities
(40-60%) for Minor Impacts for most elevations below 4,000ft
through the duration of this multi-day event. The Oregon Cascades
will likely pick up anywhere between 1-3 feet of snow through
Tuesday afternoon.

These pair of disturbances will track east into the Northern
Rockies as well. The first arrives Monday afternoon as the
divergent left-exit region of a 250mb jet streak moves over head,
helping to maximize upper level ascent. In fact, with both Pacific
systems will help to co-locate that divergent left-exit region of
the 250mb jet streak through Tuesday afternoon. With sufficient
Pacific moisture aloft and some additional upslope enhancement as
well, this will be a snowy Monday and Tuesday from the Blue
Mountains on east through the Boise, Sawtooth, Bitterroots, Lewis
Range, Absaroka, and Teton Mountains. The Blue, Sawtooth, and
Tetons are the most likely mountain ranges to see snowfall totals
of 1-2 feet with localized amounts topping 2 feet through Tuesday
afternoon.

...Central Plains into the Upper Midwest...
Day 1...

An icy wintry mix on the northern flank of a closed 500mb low will
track through portions of the Midwest this afternoon and into the
Great Lakes tonight. Modest 850mb WAA and 290K isentropic glide via
southerly low level flow will overrun a boundary layer air-mass
that contains wet-bulb temperatures at or below freezing. While
some minor snow accumulations are possible in east-central
Wisconsin, this setup favors sleet/freezing rain the most. Once the
worst of the ice concludes over eastern Iowa this evening, freezing
rain will be most problematic tonight in Michigan where surface
temperatures are coldest thanks to residual snow pack, particularly
in western Michigan following recent lake-effect snow. WPC
probabilities show low-to-moderate chances (30-50%) in parts of
western and northern Michigan through Sunday morning. A light
wintry mix is expected to linger across the northern half of
Michigan's Mitten through midday Sunday before finally concluding
Sunday evening. West-central Michigan does feature Minor Impact
potential on the WSSI due to Ice Accumulation tonight and into
Sunday morning.

...Central Appalachians...
Days 1-2...

The same closed low responsible for the icy conditions in the Great
Lakes will direct its plume of low-mid level moisture at the Mid-
Atlantic tonight and into Sunday. The Northeast and Mid-Atlantic
are both witnessing a record-breaking dome of high pressure
overhead today and into tonight that is anchoring a sub-freezing
air-mass through Sunday. Accumulating freezing rain is possible as
early as this evening in the Smokeys and Blue Ridge where easterly
low-level winds direct Atlantic moisture into these ranges, while simultaneously fostering upslope enhancement of precipitation
rates. By Sunday morning, the plume of moisture arrives in the
Central Appalachians most areas starting off as snow, but as the
burgeoning 850-750mb warm nose arrives, should force snow to
changeover to an icy mix of sleet/freezing rain from parts of
western PA on east through the Laurel Highlands and Allegheny
Mountains Sunday afternoon. Freezing Rain will be common as far
south as the Potomac Highlands and even parts of the Blue Ridge in
northern Virginia. How long freezing rain lingers is dependent
upon how long cold-air damming can remain wedged into the nooks and
crannies of the valleys of the Mid-Atlantic and Northeast. Periods
of snow are likely farther north in New York State and interior
New England while ice is possible from the Susquehanna and Delaware
Valleys to parts of the Hudson Valley Sunday night. This could
make for an icy Monday AM commute for portions of northern NJ,
southern NY (excluding NYC) and southern New England.

Latest WPC probabilities suggest low-to-moderate chances (30-50%)
for ice accumulations >0.25" in parts of western MD and into the
Laurel Highlands. It is likely that an expansive swath of minor
ice accumulations envelope much of the interior portions of he
Mid-Atlantic with the Central Appalachians most favored for
hazardous ice. From the Smokeys of NC on north along the Blue Ridge
and into the Alleghenys, WPC probabilities show moderate-to-high
chances (50-70%) for >0.1" of ice. It is worth noting that WPC
probabilities for >0.01" of ice are high (>70%) from the Catoctins
and Parr's Ridge of northern MD through southeast PA and into
northern NJ. While unlikely to cause significant problems, these
accumulations could still result in slick travel conditions for
these areas Sunday night.


...Northern Plains...
Days 1-3...

The upper level shortwave trough responsible for the excessive
rainfall and heavy snow across northern California will race
through the Intermountain West tonight and into the Northern
Plains Sunday morning. Low pressure tracking east through eastern
Montana will see weak WAA at low-levels overrun a sub-freezing air-
mass that results in minor ice accumulations in northeast Montana
and far western Montana. As the storm tracks east through North
Dakota Sunday afternoon and into northern Minnesota by early Monday
morning, the storm will strengthen and snow will wrap around the
western flank of the storm system. Snowfall rates may be generally
light, but a tightening of the pressure gradient will cause
blustery winds to take shape by Monday morning. The WSSI-P shows low-to-moderate chances (20-40%) for Minor Impacts along the
ND/northwest MN border along Canada due to a combination of Snow
Amount and Blowing Snow through Monday morning.

As this system departs, a quick moving wave of light snow may track
across South Dakota and into southern Minnesota on Tuesday. WPC
probabilities are not impressed with probabilities >10% for
snowfall totals >4" at the moment, but this setup will have some
banded precip potential along the northern flank of a warm front
and the region's location beneath the divergent left-exit quadrant
of an approaching 250mb jet streak. These setups have been known
to produce quick 1-3" swath of snowfall that could result in
locally hazardous travel conditions, so this will be monitored in
subsequent forecast cycles.


Mullinax



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FOUS11 KWBC 150727
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
227 AM EST Sun Dec 15 2024

Valid 12Z Sun Dec 15 2024 - 12Z Wed Dec 18 2024


...Pacific Northwest through the Northern Rockies...
Days 1-3...

An active period of weather will bring periods of of heavy snow to
much of the region through the middle of next week.

The period begins with a broad longwave mid-level trough centered
over the Rockies. Within this trough, a modest shortwave impulse
and accompanying vorticity maxima will be shedding northeast into
the Northern Rockies, interacting with modest downstream moisture
reflected by near-normal PWs according to NAEFS. This will spread a
swath of precipitation across ID/WY/MT the first half of D1 before
an approaching shortwave ridge brings an end to the precipitation.
Most of this precipitation will be light, with the exception likely
in the vicinity of NW WY/Tetons where upslope flow and some
enhanced fgen will drive heavier rates and greater snowfall
accumulations. WPC probabilities D1 for more than 8 inches of snow
are high, above 70%, in the Tetons and adjacent Absarokas to the
north.

Behind this first shortwave, the aforementioned shortwave ridging
will cause a brief respite to precipitation in the Pacific
Northwest, but this will come to an end by the start of D2 as
another amplified closed low drifts eastward across the Pacific
Ocean. Downstream of this low, mid-level divergence will approach
the coast Monday morning, with periodic PVA through shedding
vorticity lobes helping to enhance ascent. The downstream SW/WAA
ahead of this feature will surge moisture back onshore as well,
reflected by IVT approaching the 97th percentile as both GEFS and
ECENS probabilities for IVT of 500 kg/m/s reach 20-30%. This will
spread heavier precipitation back onshore the Pacific Northwest,
with snow falling above generally 3000 ft in the Cascades, and
spilling into the interior Northwest where snow levels will be even
lower, around 1500 ft. This will result in moderate to heavy snow
from the Shasta/Siskiyou/Klamath region of CA northward along the
Cascades and as far inland as the Blue Mountains and Salmon River
range. Across these areas, WPC probabilities are moderate to high
(50-70%) for at least 8 inches of snow, and locally more than 12
inches is likely (>70%) in the highest terrain of the OR Cascades
and near Mt. Shasta.

During D3, a warm front just offshore will lift northward, and the
accompanying precipitation will follow on enhanced WAA lifting
across OR/ID/WA. The intensity of this precipitation should
generally be lighter in the snow areas than on D2, but additional
accumulations exceeding 6 inches are possible (30-50%), highest in
the WA Cascades.


...Central Appalachians...
Day 1...

Weakening surface low pressure moving across the OH VLY will spread
downstream moisture northward into the Central Appalachians and
Mid-Atlantic States. This moisture will surge northward on
intensifying 295K isentropic ascent, lifting atop a retreating but
still expansive high pressure centered over New England and its
resultant wedge east of the Appalachians. As the high retreats and
the surface low weakens while moving eastward, low-level flow will
veer to become primarily S/SE, not ideal for locking in any low-
level cold air. This suggests that while precipitation will begin
wintry (snow/sleet/freezing rain) in most areas from the highest
elevations of SC northward, it will quickly turn to rain outside of
the higher terrain. Even in the Mid-Atlantic and Northeast as far
southeast as the I-95 corridor, precipitation may start as a brief
period of sleet or snow before turning to rain. This will limit
wintry accumulation in many areas.

However, in the higher terrain of WV, through the MD Panhandle, and
into the Laurel Highlands, a prolonged period of freezing rain is
likely, which will accrete to moderate to substantial ice amounts
through Sunday night. Some modest conditional instability reflected
by theta-e lapse rates around 0C/km will support at times briefly
heavy rain rates, somewhat limiting the accretion potential
(especially without any dry-bulbing affects), but prolonged
duration of freezing rain has still bumped WPC probabilities to
moderate (30-50%) for 0.25" centered near the MD Panhandle, with
high probabilities (>70%) for at least 0.01" stretched from NC
northward along the Appalachians into Upstate NY.


...Northern Plains...
Days 1-2...

A wave of low pressure moving from MT through MN Sunday into Monday
will deepen in response to a mid-level shortwave closing off
overhead, collocated with the LFQ of a modest but poleward arcing
jet streak. As this low slowly intensifies, the downstream plume of rich theta-e air spreading northward from the Gulf of Mexico will begin
to wrap cyclonically around the low, potentially supporting modest
TROWAL development pivoting into far northern ND and northern MN
Sunday night. The leading WAA ahead of this developing low will
likely result in some modest freezing rain across far western ND
and eastern MT early D1 due to lack of saturation within the DGZ,
but WPC probabilities on D1 are above 70% for at least some icing,
with a narrow channel of 10-30% near the ND/MT border for 0.1
inches.

More impressive is likely to be the snowfall on the northern edge
of this system as it progresses east, with snow becoming more
intense Monday from eastern ND through northern MN. Here, elevated
instability beneath the TROWAL combined with modest deformation
could result in banded snow structures, offsetting otherwise modest
omega present within the column. There is still some uncertainty
into how much snow may result as precipitation expands and
intensifies Monday, but WPC probabilities have increased and are
50-70% from northern ND across into northern MN for 2+ inches, and
the WSE plumes do suggest at least a low-end potential for 4-5
inches in some areas. Regardless of the intensity and amounts of
snow, hazardous travel is likely as fluffy SLR snow combines with
gusty winds to cause snow covered roads with restricted visibility,
but WSSI-P for moderate impacts remains quite low for this area
suggesting primarily minor impacts.


Weiss


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FOUS11 KWBC 152001
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
301 PM EST Sun Dec 15 2024

Valid 00Z Mon Dec 16 2024 - 00Z Thu Dec 19 2024


...Pacific Northwest through the Northern Rockies...
Days 1-3...

As the first storm system that brought periods of heavy snow to the
mountainous terrain of the Pacific Northwest and Northern Rockies
winds down this afternoon, the next frontal system associated with
a shortwave trough will track towards the region tonight. As the
warm front approaches, a slug of 850-700mb moisture will slam into
the Pacific Northwest coast that leads to periods of mountain snow
in the usual suspects (Olympics, Cascades, Siskiyou,
Trinity/Shasta) tonight and into Monday morning. This is no slouch
of a moisture stream either, as evident by an IVT surpassing 500
kg/m/s just off the West Coast that is above the 90th
climatological percentile per NAEFS.

This moisture will spill over into the interior Northwest and
eventually the Northern Rockies by Monday afternoon, thanks to
aforementioned IVT that by Monday morning is above the 97.5
climatological percentile as it takes aim at northern California.
In fact, 500mb mean specific humidity levels are also above the
90th climatological percentile by 00Z Tuesday across northern UT
and into the Tetons of WY. The region also lies beneath the
divergent left-exit region of a >120kt 250mb jet streak Monday
evening. Add in westerly 700-500mb winds that are 40-50 kts and
this is a good setup for upslope enhanced snowfall rates in N-S
oriented ranges (such as the Boise, Sawtooth, and Teton Mountains).
Westerlies advecting Pacific moisture into the northern Rockies
will keep snow in the forecast through Tuesday afternoon. By the
time snow concludes Tuesday night the Blue, Boise, Sawtooth, and
Teton Mountains are likely to see anywhere from 10-20 inches of
snow with totals approaching 30" in the peaks of the Tetons.

Meanwhile, the Pacific Northwest will remain mired in the same
moist westerly flow Monday night and into Tuesday as the initial
upper low opens up and tracks over head Monday night. As it heads
east on Tuesday, upper level ridging and WAA aloft ensues ahead of
the next approaching PAcific shortwave trough. The warm front
lifting north through the region Tuesday afternoon will coincide
with the arrival of strong high pressure over southwest Canada.
Sub-freezing air racing south into the Columbia Basin will make for
a classic overrunning setup that gives rise to an icy wintry mix
Tuesday afternoon and linger into Tuesday night. While WAA
increases on Wednesday ahead of the next PAcific shortwave trough,
snow levels will rise to the point where even elevations >6,000ft
will struggle to support snow. However, frozen precipitation may
linger along the northern Washington/Canada border. WSSI-P shows
low chance probabilities for Minor Impacts in parts of the Columbia
Basin due to ice Tuesday night and into Wednesday morning. Over
this prolonged stretch of heavy snow, the Cascade Range and
Olympics above 4,000ft are likely to see 12 inches at least with
the elevations above 5,000ft seeing as much as 2 feet of snow. This
includes elevations farther east concluding the Blue Mountains,
the Bitterroots in northern Idaho, and the Lewis Range in western
Montana. even the Salmon/Trinity/Shasta Mountains of northern
California above 5,000ft could pick up as much as a foot of snow.


...Central Appalachians...
Day 1...

An icy wintry mix has enveloped much of the northern Mid-Atlantic
as a potent upper low tracking across the Lower Great Lakes is
producing WAA at low-mid levels. This WAA and plume of moisture
aloft is overrunning a sub-freezing air-mass anchored by
exceptionally strong high pressure to the northeast. The upper
trough will weaken as it heads northeast this evening with
generally minor snowfall accumulations (1-3", locally up to 4") in
northern PA on east through the Poconos and Catskills tonight. The
freezing rain/sleet mix remains the most problematic when it comes
to hazardous travel from the central Appalachians this afternoon, to
portions of the Lehigh Valley, Delaware Valley, and through the
Tri-State region north of NYC tonight and into Monday morning. This
could make for a slick morning commute in parts of the Tri-State
area. Most additional ice accumulations will be <0.1" with the
Laurel Highlands featuring the better chances (30-50%) for
additional ice accumulations >0.1" through tonight. The WSSI does
show Minor Impacts in parts of central PA, the Catskills, and
northern NJ with lingering Moderate Impacts from the Laurel
Highlands on south through Garrett County, MD and the Appalachians
of West Virginia.


...Northeast & eastern Great Lakes...
Day 3...

There remains a fair amount of spread in solutions for the next
developing wave of low pressure set to track through the Ohio
Valley on Wednesday and towards the Northeast by Wednesday
evening. While track and intensity of the storm are unclear, areas
from northeast OH and northwest PA through the northern
Appalachians are currently favored for some measurable snowfall
that may lead to hazardous travel impacts Wednesday night. The
latest WPC probabilistic guidance shows low chances (10-30%) for
snowfall >4" in the Adirondacks through Wednesday evening. Guidance
trends will be closely monitored in subsequent forecasts as it does
have the potential to be a disruptive winter storm for residents in
the interior Northeast.


...Northern Plains...
Day 1...

Low pressure in North Dakota will continue to produce minor
snowfall accumulations through northern North Dakota and into
northern Minnesota tonight. As the storm strengthens overnight,
gusty winds will promote some blowing and drifting snow, but
outside of some reductions to visibility, impacts should be very
minor for these regions through Monday. WPC probabilities show
low-to-moderate chances (30-50%) for >2" of snowfall through Monday
afternoon in far northern North Dakota and northwest Minnesota.


Mullinax





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FOUS11 KWBC 160830
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
330 AM EST Mon Dec 16 2024

Valid 12Z Mon Dec 16 2024 - 12Z Thu Dec 19 2024


...Pacific Northwest through the Northern Rockies...
Days 1-3...

An active period of weather continues across the West as a trough
persists offshore shedding impulses and accompanying IVT onto the
coast. Despite this, the general trends will be for bulging
thicknesses across the West with increasingly quiet weather
forecast through the period.

On D1, a strung out vorticity lobe accompanying a shortwave trough
will lift northeast towards the WA coast, driving a frontal system
eastward towards the shore. As this feature moves to the east, it
will weaken and elongate, but still push increasing moisture into
the region within confluent mid-level flow. The highest
accompanying IVT will move across CA and into the Great Basin, but
a broader surge of elevated PWs will shift into most of the Pacific
and interior Northwest, resulting in a shield of precipitation
lifting across the area. Snow levels west of the Cascade crest will
rise to around 4000 ft, but remain much lower to the east,
especially across interior WA state at just 1500-2500 ft, but then
rise again to around 3000 ft in the Northern Rockies. Where the
best overlap of synoptic lift (through height falls, downstream
divergence, and upper level diffluence) and upslope flow combine,
likely in the Shasta/Siskiyou region, the OR Cascades, and as far
east as the Salmon Rivers and Tetons. In these ranges and above the aforementioned snow levels, WPC probabilities D1 for more than 8
inches are generally 70-90%, with locally as much as 2 feet
possible.

As this first wave sheds inland, it will be quickly followed by
another, more impressive surge of IVT (GEFS and ECENS probabilities
for >500 kg/m/s as high as 80%). This secondary plume will traverse
more quickly north as a low-level trough axis and following warm-
front extending from another approaching shortwave pushes from
northern CA into WA state. This surge will drive snow levels
rapidly upward, reaching as high as 8000 ft by the end of D2 west
of the Cascades, but even 5000-7000 ft as far east as the northern
Rockies and the Great Basin. This will cause a rapid decrease in
snow coverage while SLRs also fall to become much heavier (also
reflected by increasing snow load probabilities in the WSSI-P). WPC probabilities D2 are highest in the WA Cascades and Northern
Rockies where they reach 70-90% for an additional 6+ inches, with
lighter additional accumulations expected across the Salmon Rivers,
Sawtooth, and Tetons.

During D3 most of the ascent and accompanying moisture shifts
eastward into the High Plains, bringing a brief reprieve to
snowfall in the Northwest. However, early in the period /Wednesday
morning/ some light snow is likely in the higher terrain of the WA
Cascades, Okanogan Highlands, and Northern Rockies, where WPC
probabilities are moderate (50-70%) for 4+ inches of additional
snowfall. Potentially more problematic early D3 will be the threat
of some freezing rain along and east of the Cascades, including
some of the passes. The deterministic icing amounts from the
guidance are quite low, as are WPC probabilities for even 0.01
inches of ice (just 10-30%), but any light icing could be
problematic for the Wednesday morning commute.


...Northern Plains into the Upper Midwest...
Day 2...

A modest mid-level impulse will traverse southeast from the
Northern Rockies to the Western Great Lakes Tuesday, producing
sharp but temporally short height falls, combined with increasing
LFQ diffluence as a jet streak pivots across the region. The trends
in the guidance have been for this jet streak to be more intense
and more strongly poleward arcing, while also being a bit
displaced to the south. Although moisture downstream of this
shortwave will be modest, it will likely be effectively wrung out
as fgen intensifies directly into the deepening DGZ, supporting
snowfall rates in a narrow channel that may exceed 1"/hr. The
system is progressive, but the briefly heavy snow should cause some
moderate accumulations and modest impacts. WPC probabilities are
currently less than 10% for 4+ inches (but 50-70% for 2+ inches),
but it is possible some higher snowfall will occur with this band.


...Northeast & Eastern Great Lakes...
Day 3...

Guidance starting to converge on a solution which will bring a
progressive but impactful winter storm to the interior Northeast
Wednesday night and Thursday. Dual shortwaves, one lifting out of
the Missouri Valley Wednesday, and a second northern stream impulse
diving across the Great Lakes may interact or phase near 12Z
Thursday (end of D3) over Upstate New York. This will occur along
an eastward racing cold front, with downstream and intensifying
warm advection spreading moisture northward as an expanding
precipitation shield from the eastern OH VLY into New England.
It appears through the OH VLY and Mid-Atlantic, most of the
precipitation will be rain as the column dries before cold air can
catch the moisture.

However, across interior PA, NY, and northern New England, the
risk for heavy snow is increasing as the two shortwaves phase and
produce secondary cyclogenesis off the coast. At this time there
continues to be considerable timing and spatial spread of this
secondary low, leading to wide fluctuations in snowfall potential.
However, the setup should result in a pronounced band of heavy snow
through WAA/Fgen, resulting in an axis of heavy accumulations.
Where this is going to occur, and with what intensity, remains very
uncertain as reflected by DESI LREF plumes and very low WSSI-P
probabilities for moderate impacts. However, current WPC
probabilities have increased, and feature a 50-70% chance of at
least 4 inches of snow, greatest across the high terrain of the
Adirondacks, Greens, and Whites.


...Northern Plains...
Day 3...

Zonal flow across Western Canada will become suppressed in response
to a sharpening shortwave which will amplify as it digs east of the
Northern Rockies and reaches the Dakotas before the end of the
forecast period. This shortwave is progged to remain progressive,
but should deepen substantially and will work in tandem with a
digging jet streak to drive pronounced ascent Wednesday night into
eastern MT and the Dakotas. This synoptic lift moving atop a
baroclinic gradient along a weak front will help launch
cyclogenesis, and a strong clipper type low is likely to dive into
the region D3. Impressive WAA downstream of this feature will help
deepen the DGZ (SREF probabilities for > 50mb of depth increase to
70%), which should work together with the ageostrophic response to
the jet streak to produce a band of impressive fgen. The column is
quite cold at this time and the best fgen may reside above the
DGZ, but a band of heavy snow is becoming more likely Wednesday
night. A lot of details will still need to be ironed out, but
current WPC probabilities are moderate (50-70%) for 4+ inches of
snow across northern ND, with additional moderate to heavy snow
likely to continue downstream through D4.


Weiss



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FOUS11 KWBC 161953
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
253 PM EST Mon Dec 16 2024

Valid 00Z Tue Dec 17 2024 - 00Z Fri Dec 20 2024


...Pacific Northwest through the Northern Rockies...
Days 1-3...

20Z Update: Current forecast remains on track across the Western
CONUS with increasing snowfall coverage expanding towards the
interior west as modest jet coupling with the approach of the next
Pacific jet will punch through the PAC NW with the nose of the jet
likely encroaching the Tetons late this evening. Strong mid-level
diffluence and correlated positive vorticity advection in
association with the shortwave trough ejection correlate highly to
an enhanced region of ascent focused downstream of the Cascades
with the best opportunity for significant snowfall likely over the
Tetons and adjacent terrain. Previous WPC probs of 70-90+% for >8"
still have merit with little deviation from the previous forecast
as run-to-run continuity remains high suggesting a positive
handling by the current NWP suite of the evolving mid and longwave
pattern west of the Rockies.

Kleebauer

..Previous Discussion..

An active period of weather continues across the West as a trough
persists offshore shedding impulses and accompanying IVT onto the
coast. Despite this, the general trends will be for bulging
thicknesses across the West with increasingly quiet weather
forecast through the period.

On D1, a strung out vorticity lobe accompanying a shortwave trough
will lift northeast towards the WA coast, driving a frontal system
eastward towards the shore. As this feature moves to the east, it
will weaken and elongate, but still push increasing moisture into
the region within confluent mid-level flow. The highest
accompanying IVT will move across CA and into the Great Basin, but
a broader surge of elevated PWs will shift into most of the Pacific
and interior Northwest, resulting in a shield of precipitation
lifting across the area. Snow levels west of the Cascade crest will
rise to around 4000 ft, but remain much lower to the east,
especially across interior WA state at just 1500-2500 ft, but then
rise again to around 3000 ft in the Northern Rockies. Where the
best overlap of synoptic lift (through height falls, downstream
divergence, and upper level diffluence) and upslope flow combine,
likely in the Shasta/Siskiyou region, the OR Cascades, and as far
east as the Salmon Rivers and Tetons. In these ranges and above the aforementioned snow levels, WPC probabilities D1 for more than 8
inches are generally 70-90%, with locally as much as 2 feet
possible.

As this first wave sheds inland, it will be quickly followed by
another, more impressive surge of IVT (GEFS and ECENS probabilities
for >500 kg/m/s as high as 80%). This secondary plume will traverse
more quickly north as a low-level trough axis and following warm-
front extending from another approaching shortwave pushes from
northern CA into WA state. This surge will drive snow levels
rapidly upward, reaching as high as 8000 ft by the end of D2 west
of the Cascades, but even 5000-7000 ft as far east as the northern
Rockies and the Great Basin. This will cause a rapid decrease in
snow coverage while SLRs also fall to become much heavier (also
reflected by increasing snow load probabilities in the WSSI-P). WPC probabilities D2 are highest in the WA Cascades and Northern
Rockies where they reach 70-90% for an additional 6+ inches, with
lighter additional accumulations expected across the Salmon Rivers,
Sawtooth, and Tetons.

During D3 most of the ascent and accompanying moisture shifts
eastward into the High Plains, bringing a brief reprieve to
snowfall in the Northwest. However, early in the period /Wednesday
morning/ some light snow is likely in the higher terrain of the WA
Cascades, Okanogan Highlands, and Northern Rockies, where WPC
probabilities are moderate (50-70%) for 4+ inches of additional
snowfall. Potentially more problematic early D3 will be the threat
of some freezing rain along and east of the Cascades, including
some of the passes. The deterministic icing amounts from the
guidance are quite low, as are WPC probabilities for even 0.01
inches of ice (just 10-30%), but any light icing could be
problematic for the Wednesday morning commute.

Weiss

...Northern Plains into the Upper Midwest...
Day 2...

20Z Update: Current synoptic forecast remains steady within the
latest suite of NWP as a strong mid-level shortwave ejects into the North-Central Plains with increasing 850-700mb FGEN focused within
a skinny axis aligned northwest to southeast across central and
eastern SD into southwestern MN as we approach the end of the D2
cycle. The latest update brought up snowfall totals a bit to match
the trends of the latest QPF output from much of the deterministic
suite, as well as modest increases in the latest NBM 50th-90th
percentile for snowfall. Local bufr soundings across eastern SD
signal appreciable omega located within a marginally deep center of
DGZ with the primary time frame of interest aligning between 18-03z
during the period. A small increase in the WPC probs for >2" were
reflected in the previous forecast below with now a prob max of 80%
now forecast based off the latest forecast issuance. To add more
relevance to the setup, recent CIPS analogs for the period were
referencing high correlations to multiple events of similar
stature, most of which ended up with final results showing bands of
4-6" during past evolutions. This lends credence to the potential
for locally higher totals >4" if everything aligns within the
overlapping FGEN signatures being progged.

Kleebauer

..Previous Discussion..

A modest mid-level impulse will traverse southeast from the
Northern Rockies to the Western Great Lakes Tuesday, producing
sharp but temporally short height falls, combined with increasing
LFQ diffluence as a jet streak pivots across the region. The trends
in the guidance have been for this jet streak to be more intense
and more strongly poleward arcing, while also being a bit
displaced to the south. Although moisture downstream of this
shortwave will be modest, it will likely be effectively wrung out
as fgen intensifies directly into the deepening DGZ, supporting
snowfall rates in a narrow channel that may exceed 1"/hr. The
system is progressive, but the briefly heavy snow should cause some
moderate accumulations and modest impacts. WPC probabilities are
currently less than 10% for 4+ inches (but 50-80% for 2+ inches),
but it is possible some higher snowfall will occur with this band.

Weiss

...Northeast & Eastern Great Lakes...
Day 3...

20Z Update: Model spread has lessened compared to the previous
forecast issuance, however some differences within the low
positioning and overall 500mb evolution still maintain some
questions on the anticipated magnitude of snowfall potential across
the interior Northeast. Latest WPC probability fields for >4" and

6" have not wavered much compared to the previous forecast with

the general probability maxima tied to the higher elevations of the Adirondacks, the Green and White Mountains of VT/NH, and Western
ME. Until spread decreases, the forecast remains of medium
confidence in the total expected output, but confidence on the
location of higher snowfall relegated to the interior still remains
high due to the anticipated thermal structure situated across New
England.

Kleebauer

..Previous Discussion..

Guidance starting to converge on a solution which will bring a
progressive but impactful winter storm to the interior Northeast
Wednesday night and Thursday. Dual shortwaves, one lifting out of
the Missouri Valley Wednesday, and a second northern stream impulse
diving across the Great Lakes may interact or phase near 12Z
Thursday (end of D3) over Upstate New York. This will occur along
an eastward racing cold front, with downstream and intensifying
warm advection spreading moisture northward as an expanding
precipitation shield from the eastern OH VLY into New England.
It appears through the OH VLY and Mid-Atlantic, most of the
precipitation will be rain as the column dries before cold air can
catch the moisture.

However, across interior PA, NY, and northern New England, the
risk for heavy snow is increasing as the two shortwaves phase and
produce secondary cyclogenesis off the coast. At this time there
continues to be considerable timing and spatial spread of this
secondary low, leading to wide fluctuations in snowfall potential.
However, the setup should result in a pronounced band of heavy snow
through WAA/Fgen, resulting in an axis of heavy accumulations.
Where this is going to occur, and with what intensity, remains very
uncertain as reflected by DESI LREF plumes and very low WSSI-P
probabilities for moderate impacts. However, current WPC
probabilities have increased, and feature a 50-70% chance of at
least 4 inches of snow, greatest across the high terrain of the
Adirondacks, Greens, and Whites.

Weiss

...Northern Plains...
Day 3...

20Z Update: The overall synoptic evolution for the D3 setup across
the Northern Plains remains steady within the latest 12z model
consensus. Prospects for a heavy snowfall across north-central ND
through eastern ND into MN are increasing as run- to-run
variability has wavered little with better consensus on the axis of
impressive sfc-700mb FGEN anticipated with the approach of the
vigorous northern stream shortwave. Latest percentiles for >4" of
snowfall were updated below to include now 80% for subsequent
moderate accumulations with individual deterministic output now
pushing 10+" portions of the Upper Midwest extending over the areas
above. This threat bears monitoring for locally significant
headlines if the current trend continues.

Kleebauer

..Previous Discussion..

Zonal flow across Western Canada will become suppressed in response
to a sharpening shortwave which will amplify as it digs east of the
Northern Rockies and reaches the Dakotas before the end of the
forecast period. This shortwave is progged to remain progressive,
but should deepen substantially and will work in tandem with a
digging jet streak to drive pronounced ascent Wednesday night into
eastern MT and the Dakotas. This synoptic lift moving atop a
baroclinic gradient along a weak front will help launch
cyclogenesis, and a strong clipper type low is likely to dive into
the region D3. Impressive WAA downstream of this feature will help
deepen the DGZ (SREF probabilities for > 50mb of depth increase to
70%), which should work together with the ageostrophic response to
the jet streak to produce a band of impressive fgen. The column is
quite cold at this time and the best fgen may reside above the
DGZ, but a band of heavy snow is becoming more likely Wednesday
night. A lot of details will still need to be ironed out, but
current WPC probabilities are moderate (50-80%) for 4+ inches of
snow across northern ND, with additional moderate to heavy snow
likely to continue downstream through D4.


Weiss






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FOUS11 KWBC 170746
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
246 AM EST Tue Dec 17 2024

Valid 12Z Tue Dec 17 2024 - 12Z Fri Dec 20 2024


...Pacific Northwest through the Northern Rockies...
Days 1-2...

Shortwave ridging over the Pacific Northwest will quickly be
displaced by an approaching trough moving eastward from the
Pacific leading to height falls along the coast. While the core of
this feature will lift into British Columbia, a secondary vorticity
maxima will pivot onshore NW WA state Tuesday night with enhanced
ascent through PVA, height falls, and concurrent upper level
diffluence. This impulse will move quickly eastward, but will be
accompanied by a lead warm front and trailing cold front, causing
fluctuations in snow levels.

The heaviest precipitation is likely to accompany the warm front,
generally 18Z Tuesday through 12Z Wednesday, with a secondary surge
along the trailing cold front Wednesday morning. During the warm
frontal passage and accompanying WAA/moisture surge, snow levels
are progged to climb to as high as 8000 ft along and west of the
Cascades. This will limit significant snowfall to just the higher
terrain, with rain the primary p-type at the passes. However,
during the transition from cold to warm, a period of freezing rain
is possible even as low as Snoqualmie and Stevens Passes, creating
hazardous travel this evening before changing to all-rain. East of
the Crest, cold air locked in within Canadian high pressure will
allow for more substantial freezing rain/ice accretion through
early Wednesday. The subsequent cold front will produce primarily
rain, as snow levels crash behind it but occur with rapid drying as
well.

For the areas that get snow and ice accumulations, moderate impacts
are expected as reflected by the WSSI-P showing a high chance

80%) for moderate impacts in the WA Cascades and into the

foothills. Here, WPC probabilities for 0.1 inches of ice are
generally 10-30%, and 70-90+% for 6 or more inches of snow. By D2
as the cold front shifts east, additional moderate snowfall
accumulations are likely in the Northern Rockies where WPC
probabilities are 30-50% for 6+ inches.


...Northern Plains into the Upper Midwest...
Day 1...

A compact and fast moving shortwave will eject from the Northern
Rockies early Tuesday and then race eastward towards the Great
Lakes. This feature will remain of low amplitude, but be
accompaniedby a potent vorticity streamer to enhance otherwise
modest mid- level ascent. More impressive will be a strengthening
jet streak beginning to arc poleward immediately downstream of the
shortwave trough axis. Together this will produce an narrow
corridor of intense ascent which can support a heavy snow band
moving generally west to east from SD through southern MN and into
WI. The most intense ascent should occur during the daylight hours,
but a deepening DGZ (SREF 100mb of depth probabilities reaching
50%) which is aligned with the greatest ascent through 700-600mb
fgen, will support heavy snow rates that could exceed 1"/hr at
times. The progressive nature of this will limit snowfall totals,
but WPC probabilities for 2+ inches are above 70% in a stripe
across eastern SD into SW MN, with locally 4-6" probable as
reflected by HREF max ensemble output in the most intense snow
banding.


Days 2-3...
After this first wave exits, a brief respite will occur before a
more pronounced system digs out of Canada and dives SW into the
Northern Plains. This will be driven by a potent shortwave dropping
from the Canadian Rockies and into North Dakota by Thursday
morning, with the primary ascent efficiently overlapped with the
LFQ of a strengthening jet streak also digging into the region.
This deep layer ascent will impinge into a low-level baroclinic
boundary as a warm front drapes eastward, resulting in rapid
cyclogenesis in eastern MT, with this low then moving into ND and
then into the Great Lakes by the end of D3.

As this wave moves eastward and deepens, WAA/isentropic ascent will
begin to intensify between 280K-285K on Thursday surging moisture
into and downstream of the system, reflected by an axis of PWs
exceeding the 90th percentile according to NAEFS. At the same time,
this WAA will occur favorably into an extremely cold column to
deepen the DGZ to more than 100mb, and intensifying fgen will drive
pronounced ascent into this DGZ. The deep DGZ, a sub-DGZ isothermal
layer, and modest winds in a very cold column suggest SLRs will be
quite high, and this will rapidly accumulate despite the general
progressive nature of this system. At this time, WPC probabilities
D2 are high (70-90%) for more than 4 inches across much of northern
ND, with a stripe of 30-50% probabilities for 4+ inches extending
as far east as the Door Peninsula of WI. Locally, 6-8" of snow is
possible across ND during the event.


...Northeast & Eastern Great Lakes...
Days 2-3...

Surface low pressure will develop near the TN VLY Wednesday morning
in response to an elongated shortwave diving from the Central
Plains across the region. This shortwave will be accompanied by
modest height falls/PVA to drive ascent, and interact with the RRQ
of a distant but still noteworthy jet streak pivoting over the
eastern Great Lakes. This low will then lift northeast along a cold
front, while a secondary, and more intense, northern stream
shortwave move over the Great Lakes and into the Northeast. These
features are likely to interact across the Northeast, leading to
secondary low pressure developing off the New England coast and
deepening as it moves into Canada.

Moisture associated with these waves will surge northward on rich
theta-e advection, resulting in PWs which are progged by NAEFS to
exceed the 90th percentile in the CFSR database. This will allow
for widespread precipitation to become heavy beginning around 00Z
Thursday, with wintry precipitation spreading across interior
portions of the northeast. While there still remains some
longitudinal spread in the placement of this low as it strengthens
near New England, the ensemble clusters have begun to focus a bit
farther to the east. This will create an environment that is colder
and more supportive to wintry precipitation, especially as the low
pulls away Thursday morning. The passes of the front combined with
the isallobaric flow into the surface low will help enhance fgen as
well, which when overlapping the increased deformation NW of the
deepening low/interacting shortwaves will likely lead to some heavy
snow rates across interior New England. However, the column as a
whole still appears generally marginal for snow, so a heavy, wet
snow, with low SLRs is likely except in the highest terrain.

The progressive nature and low SLR nature of this system will limit
total snowfall amounts, but heavy accumulations are still possible,
especially in the higher terrain which could result in at least
modest impacts due to snow load. This is reflected by WPC
probabilities that feature a moderate risk (50-70% chance) of more
than 4 inches in the highest terrain of the Adirondacks, Greens,
and Whites, with lesser accumulations extending through much of
northern New England except in the lowest valleys. Locally, 12" of
snow is possible in the highest terrain of the Whites near Mt.
Washington.


Weiss


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FOUS11 KWBC 171946
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
246 PM EST Tue Dec 17 2024

Valid 00Z Wed Dec 18 2024 - 00Z Sat Dec 21 2024


...Pacific Northwest through the Northern Rockies...
Day 1...

Shortwave ridging over the Pacific Northwest will be quickly
displaced by an approaching trough moving eastward from the Pacific
leading to height falls along the coast. While the core of this
feature will lift into British Columbia, a secondary vorticity
maximum will pivot onshore NW WA state tonight with enhanced ascent
through PVA, height falls, and concurrent upper level diffluence.
This impulse will move quickly eastward, but will be accompanied by
a lead warm front and trailing cold front, causing fluctuations in
snow levels.

The heaviest precipitation is likely to accompany the warm front,
generally from now through 12Z Wednesday, with a secondary surge
along the trailing cold front Wednesday morning. During the warm
frontal passage and accompanying WAA/moisture surge, snow levels
are progged to climb to as high as 8000 ft along and west of the
Cascades. This will limit significant snowfall to just the higher
terrain, with rain the primary p-type at the passes. However,
during the transition from cold to warm, a period of freezing rain
is possible even as low as Snoqualmie and Stevens Passes, creating
hazardous travel this evening before changing to all-rain. East of
the Crest, cold air locked in within Canadian high pressure will
allow for more substantial freezing rain/ice accretion through
early Wednesday. The subsequent cold front will produce primarily
rain, as snow levels crash behind it but occur with rapid drying as
well.

For the areas that get snow and ice accumulations, moderate impacts
are expected as reflected by the WSSI-P showing a high chance

80%) for moderate impacts in the WA Cascades and into the

foothills. Here, WPC probabilities for 0.1 inches of ice are
generally 10-30%, and 70-90+% for 6 or more inches of snow.


...Great Lakes into the Northeast...
Days 1-2...

A digging trough will emerge out of Canada and dive SW into the
Midwest and Great Lakes Wednesday. It will tap into a modest plume
of Gulf moisture moving northeast up the Ohio Valley, being driven
by a separate southern stream shortwave. While the primary surface
low will track across the Mid-Atlantic and up the New England
coast and into the Canadian Maritimes, the associated area of snow
will remain well north and west of the low center, starting across
much of Michigan Wednesday morning, then intensifying over New York
and into New England Wednesday night. The aforementioned northern
stream shortwave will translate eastward along the Canadian border
with the Northeast, helping to tap the moisture plume and cause
mostly advisory level snows on the order of 3 to 6 inches for most
areas from western New York through Maine. Lower elevations with
warmer temperatures will see less snow, with higher elevations
seeing a bit more. The snow will rapidly end from west to east late
Wednesday night and Thursday morning.

...Northern Plains to the Midwest...
Days 1-3...

A potent shortwave dropping from the Canadian Rockies and into
North Dakota by Thursday morning will support weak cyclogenesis in
the form of a clipper low with the primary ascent efficiently
overlapped with the LFQ of a strengthening jet streak also digging
into the region. This deep layer ascent will impinge into a low-
level baroclinic boundary as a warm front drapes eastward,
resulting in rapid cyclogenesis in eastern MT, with this low then
moving into ND and then into the Great Lakes by the daylight hours
on D2/Thursday.

As this wave moves eastward and deepens, WAA/isentropic ascent will
begin to intensify between 280K-285K on Thursday surging moisture
into and downstream of the system, reflected by an axis of PWs
exceeding the 90th percentile according to NAEFS. At the same time,
this WAA will occur favorably into an extremely cold column to
deepen the DGZ to more than 100mb, and intensifying fgen will drive
pronounced ascent into this DGZ. The deep DGZ, a sub-DGZ isothermal
layer, and modest winds in a very cold column suggest SLRs will be
quite high, and this will rapidly accumulate despite the general
progressive nature of this system. At this time, WPC probabilities
D2 are high (70-90%) for more than 4 inches across much of northern
ND, with a stripe of 30-50% probabilities for 4+ inches extending
as far east as the Door Peninsula of WI. Locally, 6-8" of snow is
possible across ND during the event.

Once the low gets to the Midwest Thursday evening, the greatest
forcing will shift south as the northern portion of upper level
energy supporting the precipitation shield favoring the snow over
the Great Lakes shears apart and rapidly weakens. Thus, by the day
Friday, much of the lower Great Lakes (outside of the lake-effect
corridors), Appalachians, and the NY/PA border counties should only
see flurries or very light snow. Arctic high pressure quickly
building in behind this clipper will dry the atmosphere out,
resulting in very little lingering snow except for any light lake-
effect.


Wegman/Weiss


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FOUS11 KWBC 180752
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
252 AM EST Wed Dec 18 2024

Valid 12Z Wed Dec 18 2024 - 12Z Sat Dec 21 2024


...Pacific Northwest through the Northern Rockies...
Day 1...

Compact and fast moving but potent shortwave will lift eastward
from the WA coast into the Northern Rockies today through tonight.
This feature will help drive a surface low along the international
border with Canada, pushing a warm front downstream, and then
dragging a cold front from west to east in its wake. The enhanced
ascent in the vicinity of this frontal system will act upon robust
moisture (PW anomalies +2 sigma according to NAEFS) to wring out
some heavy precipitation from the Olympics through the Cascades
and into the Northern Rockies. Snow levels will steadily rise
behind the warm front, reaching as high as 8000 ft,
limiting significant snowfall to just the highest terrain, band WPC probabilities reach 50-70% for 6+ inches in the northern WA
Cascades and Northern Rockies near Glacier NP. Although snow levels
will crash behind the cold front, this will be coincident with a
rapid drying of the column, so any residual snowfall should be
light.


...Mid-Atlantic and Northeast...
Days 1-3...

A wave of low pressure will develop across the TN VLY and then race
northeast along a cold front, with secondary wave development
likely east of Maine Wednesday night. Moisture along and ahead of
this system will increase, with PWs surging towards the 90th
climatological percentile, highest east, as WAA intensifies along a
northward advancing warm front. The antecedent airmass is
marginally conducive for wintry precipitation, and without any
strong cooling from the north, locations along and east of this
low track will likely remain all rain, with just a small temporal
window for changeover back to snow as the low pulls away Thursday.

The exception will be from the Laurel Highlands through the
Adirondacks, and across much of northern New England where, despite
still a marginal atmospheric column, the precipitation should fall
primarily as snow, except in the lower valleys. A heavy and wet
(low SLR) snow is likely, which when combined with the fast
motion of this wave will keep snowfall amounts modest, and WPC
probabilities D1 above 30% for more than 4 inches of snow are
confined to the Adirondacks, Greens, Whites, and mountains of
north/central ME.

As the low moves away Thursday, some modest upslope/lake-effect
snow may occur, but in general precip intensity and coverage will
wane during Thursday. This sets the stage for an interesting
development on Friday. A shortwave digging from the Midwest and
embedded within the larger trough will sharpen and may become
negatively tilted over the Mid- Atlantic Friday, while a surface
low develops well offshore and downstream of this larger scale
trough. The guidance has trended a bit deeper with this shortwave,
leading to greater interaction with the larger low pressure
offshore. A lot of uncertainty remains, but if these systems can
interact, the overlap of moisture and some more intense ascent
due to deformation/height falls, could result in widespread, at
least light, snow across much of the Mid-Atlantic and Northeast.
Current WPC probabilities for more than 4 inches of snow are modest
at just 10-30% across much of Upstate NY, the Poconos, and eastern
MA/ME, but this event will need to be monitored with recent model
runs for the potential for heavier snowfall in some areas.


...Northern Plains to the Midwest...
Days 1-2...

A shortwave amplifying over the Canadian Rockies will dig southeast
on Wednesday moving into the Northern Plains by Thursday morning
and then continue through the Upper Midwest and then reach the
Mid-Atlantic early on Friday. As this shortwave digs and amplifies,
it will be accompanied by a sharpening jet streak to drive deep
layer ascent through overlapping height falls, PVA, and diffluence.
This synoptic lift impinging on a low-level baroclinic gradient
will result in cyclogenesis, with this low diving progressively
southeast through D1 and D2.

Downstream, moisture will begin to enhance through the region as
impressive 280-285K isentropic lift drives rich theta-e northward,
with weak TROWAL development possible on Thursday. This overlap of
moisture and ascent produces an expanding swath of snow, with the
attendant WAA surging north to deepen the DGZ while concurrently
producing a corridor of impressive fgen. The column will be
extremely cold, so the presence of a deepening DGZ with strong fgen
should cause fluffy and above-climo SLRs which will accumulate
rapidly as reflected by the WPC prototype snowband tool suggesting
1"/hr rates moving across ND and into MN. The progressive nature
of the low will somewhat minimize the potential for significant
snowfall amounts, and there remains considerable latitudinal spread
by D2, but WPC probabilities suggest a moderate risk (50-70%) in a
stripe from northern ND into eastern WI for more than 6 inches of
snow.

Weiss



$$

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FOUS11 KWBC 182006
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
306 PM EST Wed Dec 18 2024

Valid 00Z Thu Dec 19 2024 - 00Z Sun Dec 22 2024


...Mid-Atlantic and Northeast...
Days 1-3...

No big updates for the system moving through the Mid-Atlantic and
Northeast. A fast-moving low will bring a general light snowfall
from northwest PA into New England through tonight as moisture from
the Gulf interacts with a potent shortwave moving across the
Northeast. Due to its fast movement, any periods of heavy snow in
this area will be brief.

A wave of low pressure will develop across the TN Valley and then
race northeast along a cold front, with secondary wave development
likely east of Maine tonight. Moisture along and ahead of this
system will increase, with PWs surging towards the 90th
climatological percentile, highest east, as WAA intensifies along a
northward advancing warm front. The antecedent airmass is
marginally conducive for wintry precipitation, and without any
strong cooling from the north, locations along and east of this low
track will likely remain all rain, with just a small temporal
window for changeover back to snow as the low pulls away Thursday.

The exception will be from the Laurel Highlands through the
Adirondacks, and across much of northern New England where, despite
still a marginal atmospheric column, the precipitation should fall
primarily as snow, except in the lower valleys. A heavy and wet
(low SLR) snow is likely, which when combined with the fast
motion of this wave will keep snowfall amounts modest, and WPC
probabilities D1 above 30% for more than 4 inches of snow are
confined to the Adirondacks, Greens, Whites, and mountains of
north/central ME.

As the low moves away Thursday, some modest upslope/lake-effect
snow may occur, but in general precip intensity and coverage will
wane during Thursday. This sets the stage for an interesting
development on Friday. A shortwave digging from the Midwest and
embedded within the larger trough will sharpen and may become
negatively tilted over the Mid- Atlantic Friday, while a surface
low develops well offshore and downstream of this larger scale
trough. The guidance has trended a bit deeper with this shortwave,
leading to greater interaction with the larger low pressure
offshore. A lot of uncertainty remains, but if these systems can
interact, the overlap of moisture and some more intense ascent
due to deformation/height falls, could result in widespread, at
least light, snow across much of the Mid-Atlantic and Northeast.
Current WPC probabilities for more than 4 inches of snow are modest
at just 10-30% across much of Upstate NY, the Poconos, and eastern
MA/ME, but this event will need to be monitored with recent model
runs for the potential for heavier snowfall in some areas.

For the day Saturday, lingering lake-effect will persist on all of
the Lakes as Arctic high pressure builds over all the lakes, and
overhead for the upper Lakes. This very dry polar air mass will
greatly diminish the lake-effect, but due to a still very unstable
air mass between 850 temps between -15C and -20C and lake temps
around +5C, it's unlikely the lake-effect will shut off completely
until more significant WAA begins. Rather, expect light but
extremely high SLR snow in this air mass for areas mostly adjacent
to the lakes where the winds die off, while the lake-effect
continues southeastward and inland due to continued northwesterly
flow off the lower lakes.


...Northern Plains to the Midwest...
Days 1-2...

A shortwave amplifying over the Canadian Rockies will dig southeast
on Wednesday moving into the Northern Plains by Thursday morning
and then continue through the Upper Midwest and then reach the
Mid-Atlantic early on Friday. As this shortwave digs and amplifies,
it will be accompanied by a sharpening jet streak to drive deep
layer ascent through overlapping height falls, PVA, and diffluence.
This synoptic lift impinging on a low-level baroclinic gradient
will result in cyclogenesis in the form of a clipper low, which
dives southeast through D1 and D2 into the Ohio Valley.

Downstream, moisture will begin to enhance through the region as
impressive 280-285K isentropic lift drives rich theta-e northward,
with weak TROWAL development possible on Thursday. This overlap of
moisture and ascent produces an expanding swath of snow, with the
attendant WAA surging north to deepen the DGZ while concurrently
producing a corridor of impressive fgen. The column will be
extremely cold, so the presence of a deepening DGZ with strong fgen
should cause fluffy and above-climo SLRs which will accumulate
rapidly as reflected by the WPC prototype snowband tool suggesting
1"/hr rates moving across ND and into MN. The progressive nature
of the low will somewhat minimize the potential for significant
snowfall amounts, and there remains considerable latitudinal spread
by D2, but WPC probabilities suggest a moderate risk (50-70%) in a
stripe from northern ND into eastern WI for more than 6 inches of
snow.

Weiss/Wegman

$$

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FOUS11 KWBC 190754
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
254 AM EST Thu Dec 19 2024

Valid 12Z Thu Dec 19 2024 - 12Z Sun Dec 22 2024

...Midwest to Great Lakes...
Days 1-2...

A progressive Alberta Clipper producing a swath of heavy snow over
North Dakota tonight will track across the Upper Midwest today.
Modest 850mb WAA along the warm front and 850mb theta-e wrapping
around the northern flank of the storm will prompt a snow to come
down at 1-2"/hr rates in some cases from eastern ND to central
Minnesota and central Wisconsin. The higher end of those rates will
be harder to come by east of the Mississippi River as 850mb
frontolysis sets in, leading to a reduction in snowfall rates.
Still, the 500mb shortwave trough responsible for upper level
ascent will still preside over the region and allow for periods of moderate-to-heavy snow through Thursday evening. In fact, some
snowfall could be lake-enhanced along the shores of Wisconsin and
Illinois as northerly winds advecting -10C 850mb temps race over
Lake Michigan. By Thursday night, the 850mb low will track through
northern Indiana with the best accumulating snow potential to the
north of the 850mb low track. This favors central and northern
Michigan Friday morning with a chance for the Detroit metro area to
see light accumulations Friday afternoon. The storm system will
gradually weaken into an open wave Friday afternoon as it
approaches the central Appalachians, effectively ending the period
of snowfall related to this Clipper in the Great Lakes by Friday
evening.

The WSSI shows a large swath of Minor Impacts that stretch from
North Dakota and far northern South Dakota to as farther east as
northern Michigan. There are some embedded Moderate Impact areas,
primarily in parts of central North Dakota, around the Twin Cities
metro, and in eastern Wisconsin. Moderate Impacts imply these
areas can expect hazardous driving conditions with some potential
closures and disruptions to infrastructure. WPC probabilities sport
high chances (>70%) for snowfall totals >4" from central MN to the
Door Peninsula of Wisconsin. It is worth noting that there are
some localized low chance probabilities (10-30%) for snowfall
totals >8" in central MN and central WI, as well as around the
Green Bay area where lake-enhanced snowfall could result in
localized amounts approaching 10 inches.

...Central Appalachians to Northeast....
Days 2-3...

As the Alberta Clipper tracks through the Lower Great Lakes Friday
morning, a secondary 500mb vort max that raced south from south-
central Canada will begin to phase with the shortwave associated
with the Clipper. This process, starting as early Thursday night,
will lead to an amplification of the 500mb trough as it tracks
towards the Central Appalachians Friday morning. Meanwhile, off the
East Coast, increasing upper level divergence along a frontal
boundary will allow for low pressure to quickly organize and
strengthen Friday afternoon. As WAA increases along the front,
moisture will wrap around the low and lead to broad precipitation
shield just off the Northeast coast Friday night. This could result
in heavy snow along Downeast Maine, but how close to the Maine
coastline the storm gets remain lower in confidence. WPC
probabilities suggest the eastern-most portion of Downeast Maine
has low-to-moderate chances (30-50%) for snowfall >6" of snow
through Saturday evening. In addition, high pressure over Quebec
will work in tandem with the deepening storm system to tighten the
pressure gradient and strengthening low-level northerly winds off
the MA Capes. There is the potential for ocean-effect snow over
southeast MA Friday evening. WPC probabilities show low chances
(10-3) for snowfall >4", which does include the Boston metro area.

Throughout the rest of the Northeast, western NY and northern PA
will see the clipper's remnant 850mb circulation track overhead
with modest 850-700mb moisture and weak 850mb WAA will supply the
ingredients necessary for periods of snow late Thursday night and
into Friday. By Friday afternoon, the forecast over parts of the
northern Mid-Atlantic will hinge upon the strength of low-level
easterly flow and a convergence axis over the region, while
sufficient upper level divergence from the left-exit region of a
500mb jet streak arrives. Should these factors mesh together in
just the right way, a pivoting band of snow may organize from
northern MD or the northern DelMarVa Peninsula on northward
through eastern PA and into the Poconos/Catskills. The latter is
most favored for accumulating snowfall as WPC probabilities show
moderate chances (40-60%) for snowfall totals surpassing 2" between
Friday afternoon and early Saturday morning.

If there is one mountain range that is favored to witness heavier
snowfall, it is the central Appalachians in eastern WV and western
MD. As the remnant surface low of the Clipper heads east Friday
evening, its moisture source will track into the mountains at the
same time as northwesterly winds begin to increase. The end result
is upslope flow that causes periods of snow as far north as PA's
Laurel Highlands. WPC probabilities favor those windward slopes of
eastern WV with moderate chances (40-60%) for snowfall >4",
especially at elevations above 2000ft between 18Z Fri - 18Z Sat.
For all these areas mentioned in this section, the WSSI shows Minor
Impacts for impacted areas that would primarily lead to locally
hazardous travel conditions through Saturday morning.


The probabilities for significant ice accumulations are less than
10 percent.

Mullinax



$$

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FOUS11 KWBC 190856
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
356 AM EST Thu Dec 19 2024

Valid 12Z Thu Dec 19 2024 - 12Z Sun Dec 22 2024

...Midwest to Great Lakes...
Days 1-2...

A progressive Alberta Clipper producing a swath of heavy snow over
North Dakota tonight will track across the Upper Midwest today.
Modest 850mb WAA along the warm front and 850mb theta-e wrapping
around the northern flank of the storm will prompt a snow to come
down at 1-2"/hr rates in some cases from eastern ND to central
Minnesota and central Wisconsin. The higher end of those rates will
be harder to come by east of the Mississippi River as 850mb
frontolysis sets in, leading to a reduction in snowfall rates.
Still, the 500mb shortwave trough responsible for upper level
ascent will still preside over the region and allow for periods of moderate-to-heavy snow through Thursday evening. In fact, some
snowfall could be lake-enhanced along the shores of Wisconsin and
Illinois as northerly winds advecting -10C 850mb temps race over
Lake Michigan. By Thursday night, the 850mb low will track through
northern Indiana with the best accumulating snow potential to the
north of the 850mb low track. This favors central and northern
Michigan Friday morning with a chance for the Detroit metro area to
see light accumulations Friday afternoon. The storm system will
gradually weaken into an open wave Friday afternoon as it
approaches the central Appalachians, effectively ending the period
of snowfall related to this Clipper in the Great Lakes by Friday
evening.

The WSSI shows a large swath of Minor Impacts that stretch from
North Dakota and far northern South Dakota to as farther east as
northern Michigan. There are some embedded Moderate Impact areas,
primarily in parts of central North Dakota, around the Twin Cities
metro, and in eastern Wisconsin. Moderate Impacts imply these
areas can expect hazardous driving conditions with some potential
closures and disruptions to infrastructure. WPC probabilities sport
high chances (>70%) for snowfall totals >4" from central MN to the
Door Peninsula of Wisconsin. It is worth noting that there are
some localized low chance probabilities (10-30%) for snowfall
totals >8" in central MN and central WI, as well as around the
Green Bay area where lake-enhanced snowfall could result in
localized amounts approaching 10 inches.

...Central Appalachians to Northeast....
Days 2-3...

As the Alberta Clipper tracks through the Lower Great Lakes Friday
morning, a secondary 500mb vort max that raced south from south-
central Canada will begin to phase with the shortwave associated
with the Clipper. This process, starting as early Thursday night,
will lead to an amplification of the 500mb trough as it tracks
towards the Central Appalachians Friday morning. Meanwhile, off the
East Coast, increasing upper level divergence along a frontal
boundary will allow for low pressure to quickly organize and
strengthen Friday afternoon. As WAA increases along the front,
moisture will wrap around the low and lead to broad precipitation
shield just off the Northeast coast Friday night. This could result
in heavy snow along Downeast Maine, but how close to the Maine
coastline the storm gets remain lower in confidence. WPC
probabilities suggest the eastern-most portion of Downeast Maine
has low-to-moderate chances (30-50%) for snowfall >6" of snow
through Saturday evening. In addition, high pressure over Quebec
will work in tandem with the deepening storm system to tighten the
pressure gradient and strengthening low-level northerly winds off
the MA Capes. There is the potential for ocean-effect snow over
southeast MA Friday evening. WPC probabilities show low chances
(10-3) for snowfall >4", which does include the Boston metro area.

Throughout the rest of the Northeast, western NY and northern PA
will see the clipper's remnant 850mb circulation track overhead
with modest 850-700mb moisture and weak 850mb WAA will supply the
ingredients necessary for periods of snow late Thursday night and
into Friday. By Friday afternoon, the forecast over parts of the
northern Mid-Atlantic will hinge upon the strength of low-level
easterly flow and a convergence axis over the region, while
sufficient upper level divergence from the left-exit region of a
500mb jet streak arrives. Should these factors mesh together in
just the right way, a pivoting band of snow may organize some where
between northeast MD or the northern DelMarVa Peninsula on
northeast through eastern PA, the Poconos, and into the Tri-State
area. The latter is most favored for accumulating snowfall as WPC
probabilities show moderate chances (40-60%) for snowfall totals
surpassing 2" between Friday afternoon and early Saturday morning.

If there is one mountain range that is favored to witness heavier
snowfall, it is the central Appalachians in eastern WV and western
MD. As the remnant surface low of the Clipper heads east Friday
evening, its moisture source will track into the mountains at the
same time as northwesterly winds begin to increase. The end result
is upslope flow that causes periods of snow as far north as PA's
Laurel Highlands. WPC probabilities favor those windward slopes of
eastern WV with moderate chances (40-60%) for snowfall >4",
especially at elevations above 2000ft between 18Z Fri - 18Z Sat.
For all these areas mentioned in this section, the WSSI shows Minor
Impacts for impacted areas that would primarily lead to locally
hazardous travel conditions through Saturday morning.


The probabilities for significant ice accumulations are less than
10 percent.

Mullinax






$$

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FOUS11 KWBC 192036
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
336 PM EST Thu Dec 19 2024

Valid 00Z Fri Dec 20 2024 - 00Z Mon Dec 23 2024

...Midwest to Great Lakes...
Day 1...

A progressive Alberta Clipper producing a swath of heavy snow over
Wisconsin today will track into the Ohio Valley tonight before
dissipating in the central Appalachians. Some of the clipper's
energy will transfer to a developing Nor'easter well offshore of
the eastern US. Modest 850mb WAA along the warm front and 850mb
theta-e wrapping around the northern flank of the storm is
prompting the area of heavy snow from Madison to just north of
Milwaukee. The low has already begun weakening, so the area of
heaviest snow has been shrinking.

Nevertheless, some snowfall could be lake-enhanced along the
shores of Wisconsin and Illinois as northerly winds advecting -10C
850mb temps race over Lake Michigan tonight. The 850mb low will
track through northern Indiana with the best accumulating snow
potential to the north of the 850mb low track. This favors central
and northern Michigan Friday morning through the Detroit metro
area. The storm system will gradually weaken into an open wave
Friday afternoon as it approaches the central Appalachians,
effectively ending the period of snowfall related to this Clipper
in the Great Lakes by Friday evening.

The WSSI shows a large swath of Minor Impacts that stretch from
Minnesota to a small swath of northern Michigan. There are some
embedded Moderate Impact areas, primarily in central Wisconsin.
Moderate Impacts imply these areas can expect hazardous driving
conditions with some potential closures and disruptions to
infrastructure. WPC probabilities sport moderate chances (40-70%)
for snowfall totals >4" from the Door Peninsula of Wisconsin and
northeastern portions of the L.P. of Michigan north of Saginaw.

...Central Appalachians to Northeast....
Days 1-2...

As the Alberta Clipper tracks through the Lower Great Lakes Friday
morning, a secondary 500mb vort max that raced south from south-
central Canada will begin to phase with the shortwave associated
with the Clipper. This process, starting as early Thursday night,
will lead to an amplification of the 500mb trough as it tracks
towards the Central Appalachians Friday morning. Meanwhile, off the
East Coast, increasing upper level divergence along a frontal
boundary will allow for low pressure to quickly organize and
strengthen Friday afternoon. As WAA increases along the front,
moisture will wrap around the low and lead to broad precipitation
shield just off the Northeast coast Friday night. This could result
in heavy snow in Downeast Maine, but how close to the Maine
coastline the storm gets remains lower in confidence. WPC
probabilities suggest the eastern-most portion of Downeast Maine
has moderate chances (40-70%) for snowfall of 4 inches or more through
Saturday evening. In addition, high pressure over Quebec will work
in tandem with the deepening storm system to tighten the pressure
gradient and strengthening low-level northerly winds off the MA
Capes. There is the potential for ocean-effect snow over southeast
MA Friday evening. WPC probabilities show low chances (10-30%) for
snowfall >4", which does include the Boston metro area.

Throughout the rest of the Northeast, western NY and northern PA
will see the clipper's remnant 850mb circulation track overhead
with modest 850-700mb moisture and weak 850mb WAA will supply the
ingredients necessary for periods of snow late tonight and into
Friday. By Friday afternoon, the forecast over parts of the
northern Mid-Atlantic will hinge upon the strength of low-level
easterly flow and a convergence axis over the region, while
sufficient upper level divergence from the left-exit region of a
500mb jet streak arrives. Should these factors mesh together in
just the right way, a pivoting band of snow may organize somewhere
between northeast MD or the northern DelMarVa Peninsula on
northeast through eastern PA, the Poconos, and into the Tri-State
area. The latter is most favored for accumulating snowfall as WPC
probabilities show moderate chances (40-60%) for snowfall totals
surpassing 2" between Friday afternoon and early Saturday morning.

If there is one mountain range that is favored to witness heavier
snowfall, it is the central Appalachians in eastern WV and western
MD. As the remnant surface low of the Clipper heads east Friday
evening, its moisture source will track into the mountains at the
same time as northwesterly winds begin to increase. The end result
is upslope flow that causes periods of snow as far north as PA's
Laurel Highlands. WPC probabilities favor those windward slopes of
eastern WV with moderate chances (40-60%) for snowfall >4",
especially at elevations above 2000ft between 18Z Fri - 18Z Sat.
For all these areas mentioned in this section, the WSSI shows Minor
Impacts for impacted areas that would primarily lead to locally
hazardous travel conditions through Saturday morning.


The probabilities for significant ice accumulations are less than
10 percent.

Mullinax/Wegman









$$

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FOUS11 KWBC 200803
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
303 AM EST Fri Dec 20 2024

Valid 12Z Fri Dec 20 2024 - 12Z Mon Dec 23 2024

...Great Lakes...
Day 1...

The gradually weakening clipper system will track across the Lower
Great Lakes this morning with lingering periods of snow across
much of the region, although most totals will be on the light side.
As the storm tracks east towards the Mid-Atlantic, northerly winds
will accelerate over Lakes Superior, Michigan, and Huron and lead
to a handful of lake-effect streamers containing occasional bursts
of heavy snow on Friday. Latest WPC probabilities show low chances
(10-20%) for snowfall totals >4" in parts of northwest Indiana,
near Traverse City, MI, and along the coastal areas of Michigan's
thumb. Otherwise, additional snowfall totals will generally hover
around 1-3" in parts of Michigan, Indiana, and western Ohio.

...Central Appalachians to Northeast....
Days 1-2...

As the remnant circulation of the weakening clipper system
approaches the central Appalachians, its residual moisture will
flow directly into the mountain range Friday evening. By Friday
night, as low pressure off the East Coast deepens and high pressure
builds in from the north, low-level NW winds will be favorably
oriented into the Central Appalachians with some periods of snow as
far south as the Smokeys. Upslope ascent will continue into the
central Appalachians through Friday night and gradually taper off
by Saturday afternoon. Snowfall totals in east-central WV will
likely range between 1-4" with localized totals up to 6" possible
in the tallest peaks of the WV Appalachians and the Smokeys.

Farther east, an area of low pressure will organize along a
strengthening frontal boundary off the East Coast this morning.
This low will strengthen as low-level WAA to its north and a jet
streak couplet takes shape over the Gulf Stream. Meanwhile, broad
upper level divergence out ahead of the 250-500mb mean trough axis
will supply sufficient ascent aloft in tandem with the residual
moisture from the clipper system to support periods of snow over
northern PA and western NY. Colder temperatures aloft supporting
higher SLRs will also support As the low off the East Coast
deepens, easterly flow is expected to strengthen and a low level
convergence trough will setup over the northern Mid-Atlantic coast.
Latest CAMs guidance is keying on this convergence boundary
setting up over the Delaware Valley and stretching north into the
Poconos. WPC probabilities do show moderate chances (40-60%) for
snowfall >4" in parts of the Poconos, while there are low-to-
moderate chances (30-50%) for snowfall >2" in parts of the Lehigh
Valley, Delaware Valley, and into northern New Jersey. Localized
amounts in the highest elevations of the Poconos have low chances
(10-20%) for snowfall amounts topping 6". There is a chance for
minor accumulations (coating-2") along I-95 from the Philadelphia
metro on north through the NYC and Tri-State metro regions this
afternoon

Coastal New England sports the higher "boom" scenarios given the
combination of ocean-enhanced snowfall over eastern MA Friday
afternoon and the deformation axis of the coastal storm extending
far enough west to produce heavy snow over Downeast Maine Friday
night into early morning Saturday. Latest guidance is starting to
come into better agreement with a swath of 1-3" of snowfall over
eastern MA with some CAMs suggesting higher end totals (>4") within
the range of possibilities. Downeast Maine could feature the
heaviest snowfall for the event in coastal New England with WPC
probabilities showing moderate-to-high chances (50-70%) for
snowfall >4" through Saturday morning.

...Pacific Northwest...
Days 2-3...

A pair of Pacific storm systems will bring rounds of high
elevation mountain snow to the Olympics, Cascades, Blue, and
Sawtooth Mountains this weekend. Snow levels will generally be as
low as 4,000ft in the Cascades, but the heavier snowfall totals

6") will likely be confined to elevations >5,000ft in the

Olympics and Cascades. In the Blue and Sawtooth Mountains, >6"
snowfall totals will likely be confined to elevations >6,000ft.
These heavier totals will generally be above pass level in these
mountain ranges, so expect the more hazardous travel impacts to be
in the more remote and complex terrain of these mountain ranges
this weekend.


The probabilities for significant ice accumulations are less than
10 percent.


Mullinax




$$

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FOUS11 KWBC 202036
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
336 PM EST Fri Dec 20 2024

Valid 00Z Sat Dec 21 2024 - 00Z Tue Dec 24 2024


...Central Appalachians to Northeast....
Days 1-2...

As the remnant circulation of the weakening clipper system
approaches the central Appalachians, its residual moisture will
flow directly into the mountain range this evening. By tonight, as
low pressure off the East Coast deepens and high pressure builds
in from the north, low-level NW winds will be favorably oriented
into the Central Appalachians with some periods of snow as far
south as the Smokeys. Upslope ascent will continue into the central Appalachians through tonight and gradually taper off by Saturday
afternoon. Snowfall totals in east-central WV will likely range
between 1-4" with localized totals up to 6" possible in the tallest
peaks of the WV Appalachians and the Smokeys. WPC probabilities
show the greatest (30-50%) chances of 4 inches or more of snow east
of the Mississippi River associated with this current clipper
system will be across east-central WV for the Day 1 period.

Farther east, an area of low pressure will organize along a
strengthening frontal boundary off the East Coast this morning.
This low will strengthen as low-level WAA to its north and a jet
streak couplet takes shape over the Gulf Stream. Meanwhile, broad
upper level divergence out ahead of the 250-500mb mean trough axis
will supply sufficient ascent aloft in tandem with the residual
moisture from the clipper system to support periods of snow over
northern PA and western NY. As the low off the East Coast deepens,
easterly flow is expected to strengthen and a low level
convergence trough will set up over the northern Mid-Atlantic
coast. Latest CAMs guidance is keying on this convergence boundary
setting up over the Delaware Valley and stretching north into the
Poconos. Much of the snow in this region should diminish by
Saturday morning.

Coastal New England sports the higher "boom" scenarios given the
combination of ocean-enhanced snowfall over eastern MA Friday
afternoon and the deformation axis of the coastal storm extending
far enough west to produce heavy snow over Downeast Maine Friday
night into early morning Saturday. Latest guidance is starting to
come into better agreement with a swath of 1-3" of snowfall over
eastern MA with some CAMs suggesting higher end totals (>4") within
the range of possibilities. Downeast Maine will feature the
heaviest snowfall for the event in coastal New England with the
latest WPC probabilities showing high chances (70-90%) for
snowfall of 4 inches or more through Saturday afternoon. Areas
around Eastport, ME have a 30-50% chance of 8 inches or more of
snow through Saturday afternoon.

...Pacific Northwest...
Days 2-3...

A pair of Pacific storm systems will bring rounds of high
elevation mountain snow to the Olympics, Cascades, Blue, and
Sawtooth Mountains this weekend. Snow levels will generally be as
low as 4,000ft in the Cascades, but the heavier snowfall totals

6") will likely be confined to elevations >5,000ft in the

Olympics and Cascades. In the Blue and Sawtooth Mountains, >6"
snowfall totals will likely be confined to elevations >6,000ft.
These heavier totals will generally be above pass level in these
mountain ranges, so expect the more hazardous travel impacts to be
in the more remote and complex terrain of these mountain ranges
this weekend.

...Upper Midwest...
Day 3...

A low that will eject out of the northern Rockies and into the
Northern Plains on Sunday will begin to tap much colder air as it
also draws a bit of Gulf moisture north into the Upper Great Lakes
Sunday night into Monday. As the shortwave trough forcing the low
goes from being positively tilted to neutral, the increasing
divergence could cause a narrow area of snow to break out from
northern Minnesota through much of northern Michigan. The heaviest
snow totals from this low will likely fall across the eastern U.P.,
where lake-enhancement could locally increase snowfall totals. The
heaviest snows will be from late Sunday night through Monday
morning. Expect 4 to 6 inches of snow to fall across the eastern
U.P. and far northern L.P. of Michigan, with lesser amounts
elsewhere. As is typical, locally heavier snow totals are possible
where heavier and more persistent bands of snow set up, especially
if the lakes add moisture to the atmosphere.


The probabilities for significant ice accumulations are less than
10 percent.


Mullinax/Wegman







$$

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FOUS11 KWBC 210737
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
237 AM EST Sat Dec 21 2024

Valid 12Z Sat Dec 21 2024 - 12Z Tue Dec 24 2024


...Northeast...
Day 1...

Periods of snow on the backside of a departing winter storm
heading for Nova Scotia is expected to linger over Downeast Maine through Saturday morning before finally concluding Saturday afternoon. WPC probabilities show low chances (10-30%) for additional snowfall
totals >2" through Saturday morning. Farther west, cyclonic flow
over the Great Lakes will keep some lake-enhanced bands lingering
through the day with additional snowfall amounts of 1-3" expected
in parts of northeast OH, near the Finger Lakes of NY, and as far
south as the central Appalachians. Snow showers will taper off by
Saturday evening as high pressure builds in from the west.

...Pacific Northwest & Northern Rockies...
Days 1-3...

A trio of Pacific storm systems will escort rounds of Pacific
moisture into the Olympics and Cascades into early next week. Some
of this moisture will spill over into the Northern Rockies. Snow
will generally be confined to the higher elevations of these
mountain ranges due to flood of Pacific air infiltrating much of
western North America and keeping any frigid Canadian air-mass
intrusions at bay. For the Olympics and Cascades, the heaviest
snowfall is likely to be located at/above 4,500ft in elevation. The
first storm system arrives Saturday morning with locally heavy
snowfall possible as far south as the tallest peaks of northern
California. The heaviest snowfall from this event for the Cascades,
Olympics, and Blue Mountains comes Saturday afternoon and tapers
off Saturday night thanks to the storm's progressive movement.
Following a brief break Sunday morning, the next round of snowfall
arrives Sunday afternoon in the Cascades with the heaviest
snowfall occurring Sunday night. Then, following another break
during the day Monday as high pressure briefly builds in, snow
returns to the Olympics and Cascades Monday night. Guidance
suggests sharper height falls aloft and a weak CAD signature in
the Columbia Basin that may result in lower elevations snow/ice
east of the Cascades. Through 12Z Tuesday, WPC probabilities show
high chances (>70%) for snowfall >8" in elevations >5,000ft in
parts of the Olympics, Cascades, and as far east as the Blue
Mountains.

Some of this Pacific moisture will also result in high elevation
snow in parts of the Bitterroots, Sawtooth, Boise, Absaroka, Lewis, and
Teton Mountains. Of the ranges referenced, the Sawtooth and Teton
Ranges above 7,000ft feature moderate-to-high chances (50-70%) for
snowfall totals >8" through early Tuesday morning.


...Northern Plains to the Great Lakes...
Days 2-3...

The first in a series of Pacific Northwest storm systems will make
its way into Montana while the divergent left-exit region of a
110kt 250mb jet streak moves in over the Northern Plains. Southerly
flow at the lowest levels of atmosphere will intersect a frontal
boundary that gives way to weak vertical ascent over parts of North
Dakota and central Minnesota. The moisture source and DGZ aloft is
marginal, while 850mb temps rise >0C despite surface temps
remaining below freezing. This should lead to light icing
accumulations from northeast Montana and northern North Dakota to
central Minnesota Sunday afternoon and into Sunday night.

By Monday morning, a more consolidated surface low will form in
the Upper Mississippi Valley at the same time 925-850mb moisture
(with origins out of the Gulf of Mexico) streams into the Great
Lakes. 850mb WAA over a boundary layer air-mass that is cold and
quite dry will support wet-bulb temperatures that support mostly
snow from the Michigan U.P. to the northern half of Michigan's
Mitten. Broad 250-500mb troughing will also provide adequate lift
atop the atmosphere, giving rise to a more solid shield of
snowfall over the region. WPC probabilities show moderate chances
(40-60%) for snowfall totals >4" in the far northern portions of
Michigan's Mitten and in the eastern-most areas of Michigan's U.P.
In fact, there are some low-chance probabilities (10-30%) for
localized amounts >8" in the eastern Michigan U.P.

This same moisture source, synoptic-scale forcing, and isentropic
glide will translate over the eastern Great Lakes Monday night
with potentially 1-3" of snowfall along the Chautauqua Ridge and
along the Tug Hill. Localized amounts could top 4" along the Tug
Hill where some upslope enhancement would allow for slightly
heavier snowfall rates and thus higher totals than their neighbors
in western NY through Tuesday AM.


The probabilities for significant ice accumulations are less than
10 percent.


Mullinax




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FOUS11 KWBC 212029
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
329 PM EST Sat Dec 21 2024

Valid 00Z Sun Dec 22 2024 - 00Z Wed Dec 25 2024


...California, Pacific Northwest & Northern Rockies...
Days 1-3...

A particularly active pattern over the north Pacific continues
through the week ahead with a weakening wave over the interior
Northwest this evening, the next wave reaching the PacNW coast late
Sunday, followed by a more potent trough that arrives Monday night
and digs down the CA coast Tuesday.

Snow levels will be around 4000ft tonight as the ongoing wave
lifts across the northern Rockies. Day 1 PWPF are 30-60% for an
additional 6" for the ridges in and around Glacier NP.

A lull under brief ridging can be expected late tonight into Sunday
before a moisture plume ahead of the next wave reaches the PacNW
coast around midday. Snow levels surge above 6000ft with this plume
and generally drop to around 5000ft under the trough axis late
Sunday night when precip begins to taper off as flow backs
southerly. Day 2 PWPF for >6" are generally limited to the
northern WA Cascades, though they are 50-80% for the Tetons and
immediate ranges to the south as sufficient moisture streams inland
with the wave.

A potent atmospheric river arrives Monday early evening with a
focus on far northern CA. Snow Levels understandably surge up over
8000ft over the CA and southern OR Cascades Monday evening, but
crash below 5000ft Tuesday morning under the trough axis that digs
down to the central CA coast before rushing onshore Tuesday
afternoon. A wave of heavy precip shifts south down the Sierra
Nevada on Tuesday with snow levels generally dropping to 7000ft
during times of heaviest rates. Day 3 PWPF for >8" are 30-70% in
the higher WA Cascades and the High Sierra.


...Northern Plains, Great Lakes, to Northeast...
Days 1-3...

First, ongoing LES in NNWly flow off the eastern Great Lakes will
slowly diminish overnight as flow gradually weakens as high
pressure shifts over central Ontario. Day 1 PWPF for an additional
4" after 00Z are 20-50% mainly off central and eastern Lake
Ontario.

A wave over the interior Northwest this afternoon develops into a
mid-level low over north-central MT tonight in the left- exit
region of a 110kt jet streak. Southerly low level ahead of the wave
will intersect a frontal boundary and spread a wintry mix from
northeast MT across much of ND through Sunday and then central
MN/northern WI Sunday night. The moisture source and DGZ aloft are
marginal, while 850mb temps rise >0C despite surface temps
remaining below freezing. Expect a swath of light icing
from northeast Montana and northern North Dakota to central
Minnesota with PWPF for >0.1" around 10% in northwest ND. Narrow
bands of locally moderate snow can be expected north of the
surface wave and the mix and may produce an inch or two of
accumulation in spots through Sunday night.

By Monday morning, a more consolidated surface low will form in
the Upper Mississippi Valley at the same time 925-850mb moisture
(with origins out of the Gulf of Mexico) streams into the Great
Lakes. 850mb WAA over a boundary layer air-mass that is cold and
quite dry will support wet-bulb temperatures that support mostly
snow from northern Wisconsin/the Michigan U.P. to the northern
half of Michigan's Mitten. Broad 250-500mb troughing will also
provide adequate lift, giving rise to a more solid shield of
snowfall over the region. Day 2 WPC probabilities for >4" remain
moderate (40-60%) in the northern portions of Michigan's Mitten
and in the eastern third of the U.P.

Monday night, this wave shifts east over the Northeast with 1-2"
of synoptically driven (not LES) likely for areas east of Lake
Erie over the eastern Great Lakes Monday night with potentially
1-3". Snow spreads Monday evening over much of the rest of NY and central/northern New England. The progressive nature keeps max
totals limited with Day 3 PWPF for >4" generally 20-40% over the
Adirondacks, Greens and Whites as well as Down East Maine. The
highest probabilities are around 50% in the Tug Hill region where
upslope effects and some lingering LES maximize snow there. The
surface low develops a bit over the Gulf of Maine, so interests in
coastal Maine should watch for banding snow (like last night) as
the system pulls away Tuesday morning.


Jackson


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FOUS11 KWBC 220800
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
300 AM EST Sun Dec 22 2024

Valid 12Z Sun Dec 22 2024 - 12Z Wed Dec 25 2024


...California, Pacific Northwest, & Intermountain West...
Days 1-3...

A pair of atmospheric rivers (AR) are going to make for an active
multi-day stretch of weather from California on north through the
Northwest and into the Rockies. The first AR arrives Sunday
afternoon, which NAEFS shows will top 750 kg/m/s off the Oregon
coast Sunday morning, will weaken on approach but still deliver

90th climatological percentile precipitable water values into the

northwestern U.S.. This AR will neither be accompanied by an
unusually cold air-mass, nor will it be directed at a continental
polar air-mass over the Northwest. Snow levels will initially be
as low as 4,000ft in the Cascades and Olympics, but rise to above
5,000ft by Sunday night as WAA increases aloft. This same streams
of moisture will advance inland through the Northern Rockies Sunday
night and into Monday with locally heavier snowfall possible in the

5,000ft peaks of the Blue Mountains, above 7,000ft in the Salmon

River and Sawtooth Mountains of Idaho, and the Tetons in Wyoming.
The peaks of the Tetons above 8,000ft sport moderate-to-high
chances (50-70%) for snowfall totals >6", while the Blue, Salmon
River, and Sawtooth show similar probabilities for >4" through
Monday evening.

By Monday afternoon, the next AR is already getting ready to strike
the Pacific Northwest coastline, but this AR is noticeably stronger
than the one arriving late Sunday. NAEFS shows a larger 750 kg/m/s
fetch (above the 99th climatological percentile off the northern
CA coast) with origins stemming out of the subtropical Pacific. The
initial round of precipitation arriving Monday evening will be
primarily rain in western OR and northern CA (snow levels as high
as 7,000ft), but snow will be most likely in the Cascades and Blue
Mountains. By Tuesday morning, sharper height falls from the
approaching upper trough will force snow levels to drop to as low
as 4,000ft in the Cascades, Olympics, and Blue, while farther
south, snow levels fall to ~5,000ft in the CA ranges and northern
Great Basin. Unlike Sunday's AR, Tuesday's AR will have a higher
longevity with >90th climatological percentile IVTs extending as
far inland as the Wasatch and on south to the Mogollon Rim Tuesday
night. Mountain snow will be common throughout much of the
Intermountain West Tuesday night with heavy snow advancing into the
Tetons, Uinta, and Wasatch by early Wednesday morning.

WPC probabilities depict high chances (>70%) for snowfall >6" for
the central Sierra Nevada above 7,000ft. Farther north, moderate
chances (40-60%) for snowfall >4" are shown in the higher terrain
of the Blue, Salmon River, and Sawtooth ranges. Lastly, the Wasatch
and Tetons show low chances (10-30%) for snowfall >4" above
8,000ft through 12Z Wednesday, but additional snowfall is possible
Wednesday afternoon. In total through 12Z Wednesday, there are high
chances (>70%) for snowfall totals >12" for elevations above
5,000ft in the Cascades, while similar high chance probabilities
for >8" of snow are present in the Blue, Salmon River, Sawtooth,
Tetons, and central Sierra Nevada.


...Northern Plains, Great Lakes, to Northeast...
Days 1-3...

A progressive 500mb shortwave trough over western Montana this
morning will be the catalyst for the next winter storm to track
through the Midwest today, the Great Lakes on Monday, and the
Northeast Monday night into Tuesday morning. The initial wave of
low pressure will be quite weak on Sunday, but modest upper level
ascent aided by a 110kt 250mb jet streak's left-exit region over
the Northern Plains. In addition, the moisture source and DGZ are
marginal while low level temperatures above the surface are >0C.
This is an icy wintry mix setup from northeast Montana through
central North Dakota and into central Minnesota Sunday and into
Sunday night. WPC probabilities shows moderate-to-high chances for
ice accumulations >0.01" for much of central North Dakota. The
potential for light icing extends as far east as eastern Wisconsin,
northern Illinois, southern Michigan, and northern Indiana by
Monday morning.

Monday morning is when the storm system tracking through the Great
Lakes becomes more organized, while at the same time generating a

300 kg/m/s IVT (>90th climatological percentile values via NAEFS)

oriented at Michigan and the eastern Great Lakes. An expansive
area of 850-700mb WAA over a boundary layer air-mass that remains
quite cold and dry allows wet- bulb temperatures to support snow
being the primary precipitation type for much of Michigan's Mitten
and the eastern Michigan U.P.. Combined with excellent upper level
divergence courtesy of a broad 250-500mb trough to the west, and
there is likely to be a swath of heavy snow over northern Michigan
and the far eastern Michigan U.P.. Residents in souther Michigan
should anticipate some wintry precipitation, although it remains
unclear if snow or a wintry mix (causing light ice accumulations
in the process) will be dominant precipitation types. Latest WPC
probabilities show moderate-to-high chances (50-70%) for snowfall
totals >4"in northern Michigan and over the thumb of Michigan,
while some parts of northern Michigan also have moderate chances
(40-60%) for localized totals surpassing 6" through Monday
afternoon.

This same storm system will direct its anomalous moisture source
and modest low-level WAA into the Northeast Monday night.
Sufficient upper-level divergence provides support for the healthy
shield of precipitation throughout the region, while the air-mass
remains quite cold and dry ahead of the storm (temps in the teens,
dew points in the single digits). Higher SLRs of 12-16:1 are
anticipated in these areas and the ground is very cold, which
combined with the event occurring at night, will maximize snowfall
accumulation potential. The storm races off over Nova Scotia by
midday Tuesday and snowfall should taper off by Tuesday afternoon.
WPC probabilities do show moderate-chance probabilities (40-60%)
for snowfall >4" in parts of northern NY (including the Tug Hill)
and in the tallest peaks of the Green mountains. Should guidance
continue to trend wetter, it is possible for an increase in
snowfall amounts in subsequent forecast. Downeast Maine also sports
similar moderate-chance probabilities for >4" of snowfall through
Tuesday morning. For those affected areas listed above, the WSSI-P
does sport moderate-chance probabilities (40-60%) for Minor Impacts
from northern Michigan and northern NY to Downeast Maine.


Mullinax



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FOUS11 KWBC 222036
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
336 PM EST Sun Dec 22 2024

Valid 00Z Mon Dec 23 2024 - 00Z Thu Dec 26 2024


...California, Pacific Northwest, & Intermountain West...
Days 1-3...

As the first of a pair of atmospheric rivers (AR) causing active
weather continues moving ashore in northern CA, OR, and WA,
significant precipitation associated therewith should diminish into
tonight as the moisture plume shears apart over the interior
Pacific Northwest. GEFS integrated vapor transport (IVT) analysis
shows this first AR has topped at about 600 kg/m/s off the Oregon
coast this morning. This AR will neither be accompanied by an
unusually cold air-mass, nor will it be directed at a continental
polar air-mass over the Northwest. Snow levels are currently over
7,000ft as per 18Z NBM analysis from the Cascades west. As the
precipitation moves inland, snow levels will remain above 6,000 ft
into Idaho and the interior Northwest. These very high snow levels
will confine any significant snowfall through Monday evening to the
highest peaks of the northern Washington Cascades and the Blue,
Salmon River, Tetons, and Sawtooth Ranges. The peaks of the Tetons
above 8,000ft sport moderate- to- high chances (50-70%) for
snowfall totals >6", while the Blue, Salmon River, and Sawtooth
show similar probabilities for >4" through Monday evening.

By Monday afternoon, the next AR will be moving into the Pacific
Northwest coastline, especially northern California. This AR is
noticeably stronger than the one moving ashore now. NAEFS shows a
larger 750 kg/m/s fetch (above the 99th climatological percentile
off the northern CA coast) with origins stemming out of the
subtropical Pacific. The initial round of precipitation arriving
Monday evening will be primarily rain in western OR and northern CA
(snow levels as high as 7,000ft), but snow will be most likely in
the Cascades and Blue Mountains. By Tuesday morning, sharper height
falls from the approaching upper trough will force snow levels to
drop to as low as 4,000ft in the Cascades, Olympics, and Blue,
while farther south, snow levels fall to ~5,000ft in the CA ranges
and northern Great Basin. Unlike today's AR, Tuesday's AR will
have a higher longevity with >90th climatological percentile IVTs
extending as far inland as the Wasatch and on south to the Mogollon
Rim Tuesday night. Mountain snow will be common throughout much of
the Intermountain West Tuesday night with heavy snow advancing
into the Tetons, Uinta, and Wasatch by early Wednesday morning.

WPC probabilities depict high chances (>70%) for snowfall >6" for
the central Sierra Nevada above 7,000ft. Farther north, moderate
chances (40-60%) for snowfall >4" are shown in the higher terrain
of the Blue, Salmon River, and Sawtooth ranges. Lastly, the Wasatch
and Tetons show low chances (10-30%) for snowfall >4" above
8,000ft through 12Z Wednesday, but additional snowfall is possible
Wednesday afternoon. In total through 12Z Wednesday, there are high
chances (>70%) for snowfall totals >12" for elevations above
5,000ft in the Cascades, while similar high chance probabilities
for >8" of snow are present in the Blue, Salmon River, Sawtooth,
Tetons, and central Sierra Nevada. Snow will continue into Utah and
Colorado through the day on Wednesday with a 30-50% chance of 3-6
inches of snow through Wednesday night.


...Northern Plains, Great Lakes, to Northeast...
Days 1-3...

A progressive 500mb shortwave trough over western Montana this
morning will be the catalyst for the next winter storm to track
through the Great Lakes on Monday and the Northeast Monday night
into Tuesday morning. The initial wave of low pressure is quite
weak, with modest upper level ascent aided by a 110kt 250mb jet
streak's left-exit region over the Northern Plains. In addition,
the moisture source and DGZ are marginal while low level
temperatures above the surface are above freezing. This is an icy
wintry mix setup from northeast Montana through central North
Dakota and into central Minnesota into tonight. WPC probabilities
shows moderate-to-high chances for ice accumulations >0.01" for
much of west-central North Dakota. The potential for light icing
extends as far east as eastern Wisconsin, northern Illinois,
southern Michigan, and northern Indiana by Monday morning.

Monday morning is when the storm system tracking through the Great
Lakes becomes more organized, while at the same time generating a

300 kg/m/s IVT (>90th climatological percentile values via NAEFS)

oriented at Michigan and the eastern Great Lakes. An expansive
area of 850-700mb WAA over a boundary layer air-mass that remains
quite cold and dry allows wet- bulb temperatures to support snow
being the primary precipitation type for much of Michigan's Mitten
and the eastern Michigan U.P.. Combined with excellent upper level
divergence courtesy of a broad 250-500mb trough to the west, and
there is likely to be a swath of heavy snow over northern Michigan
and the far eastern Michigan U.P.. Residents in southern Michigan
should anticipate some wintry precipitation, although it remains
unclear if snow or a wintry mix (causing light ice accumulations
in the process) will be dominant precipitation types. The latest
WPC probabilities show high chances (70-90%) for snowfall totals

4" in northern Michigan and over the thumb of Michigan, while some

parts of northern Michigan also have moderate chances (40-60%) for
localized totals surpassing 6" through Monday afternoon.

This same storm system will direct its anomalous moisture source
and modest low-level WAA into the Northeast Monday night.
Sufficient upper-level divergence provides support for the healthy
shield of precipitation throughout the region, while the air-mass
remains quite cold and dry ahead of the storm (temps in the teens,
dew points in the single digits). Higher SLRs of 12-16:1 are
anticipated in these areas and the ground is very cold, which
combined with the event occurring at night, will maximize snowfall
accumulation potential. The storm races off over Nova Scotia by
midday Tuesday and snowfall should taper off by Tuesday afternoon.
WPC probabilities show moderate-chance probabilities (40-60%) for
snowfall >4" in parts of northern NY (including the Tug Hill) and
in the tallest peaks of the Green mountains. Should guidance
continue to trend wetter, it is possible for an increase in
snowfall amounts in subsequent forecast. Downeast Maine also sports
similar moderate-chance probabilities for >4" of snowfall through
Tuesday morning. For those affected areas listed above, the WSSI-P
does sport moderate-chance probabilities (40-60%) for Minor Impacts
from northern Michigan and northern NY to Downeast Maine.


Wegman/Mullinax






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FOUS11 KWBC 230757
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
257 AM EST Mon Dec 23 2024

Valid 12Z Mon Dec 23 2024 - 12Z Thu Dec 26 2024


...The West...
Days 1-3...

An active pattern continues this week with mountain snow expected
from the West Coast mountain ranges to both the Northern and
Central Rockies. This stormy pattern in the short range is due to
three atmospheric rivers (ARs) bringing plume after plume of
Pacific moisture into the western U.S.. The first AR will gradually
weaken throughout the remainder of the day, but residual Pacific
moisture and a lack of a meaningfully cold air will keep most
heavy snowfall in the Northern Rockies above 7,000ft. Snow should
taper off over the Northern Rockies by Monday evening.

The second AR arrives Monday evening with IVT values topping 1,000
kg/m/s off the Oregon coast and moisture origins stemming out of
the subtropical Pacific. The initial round of precipitation
arriving Monday afternoon will be primarily rain in northern CA,
western OR, and western WA as snow levels in the OR Cascades are as
high as 8,000ft and even topping 9,000ft in northern CA. The
Cascades and Blue Mountains will have the better odds of staying
mostly snow above 5,000ft. By Tuesday morning, sharper 700-500mb
height falls from the approaching upper trough will force snow
levels to drop as low as 3,000ft at pass level in the WA Cascades
and below 6,000ft in the Sierra Nevada by Tuesday afternoon. The
Sierra Nevada have the best odds for an impactful snowfall event
with the Winter Storm Outlooks now sporting 50% odds of snowfall
exceeding warning criteria for elevations >7,000ft. Periods of
high elevation snow in the Great Basin and Wasatch are likely
Tuesday night but heavier accumulations (>4") will be mostly
confined to elevations at/above 8,000ft. Through 12Z Wednesday, WPC probabilities show moderate-to-high chances (50-70%) for snowfall

8" above 7,000ft in the Sierra Nevada, above 5,000ft in the Cascades/Olympics/Blue Mountains, and above 8,000ft in the Tetons.

The third and more expansive AR arrives Wednesday as a powerful
sub-960mb low tracks towards the British Columbia coast. NAEFS
shows this AR with moisture origins northwest of Hawaii that will
be directed at the Pacific Northwest, including IVT values topping
the 97.5 climatological percentile. Unlike the first pair of ARs,
this one will have a slightly colder air-mass to work with at the
onset while mean 700-300mb winds out of the WSW are better aligned
orthogonally to enhance upslope ascent into the Olympics and
Cascades. This is a recipe for heavy/wet snow in the Olympics and
Cascades Wednesday afternoon and into Wednesday night. Latest WPC
probabilities show high chances (>70%) for >8" of snowfall for
elevations >3,000ft, and similar chances for >12" of snowfall
above 5,000ft. The WSSI-P does show 50-70% odds for Moderate
Impacts in these ranges through Thursday AM, which would include
some of the WA Cascade passes.


...Northern Plains, Great Lakes, to Northeast...
Days 1-2...

A positively tilted 250-500mb trough over the Midwest this morning
is providing sufficient upper-level ascent over the Great Lakes and
supporting a weak area of low pressure tracking towards southern
Wisconsin. Broad 850mb WAA and 290K isentropic glide via SWrly flow
will introduce a slug of Gulf of Mexico moisture (embedded within a

300 kg/m/s IVT) into the region which maintains sub-freezing

boundary layer temperatures as the precipitation arrives. This
will result in periods of snow on the north side of the low that
stretches from southeast MN and central WI to northern MI today and
into tonight. WPC probabilities show moderate-to-high chances
(50-70%) for snowfall totals >4" across the northern tier of
Michigan's Mitten with low chances (10-30%) or >6". Some light
icing is possible in parts of southeast MN, western WI, southern
MI, and northern IN where there are moderate-to-high chances
(50-70%) for ice accumulations >0.01". With ground temperatures so
cold, even minor amounts <0.1" can cause slick conditions on
roadways.

As the storm heads east Monday night, storm system will direct its
anomalous moisture source and modest low-level WAA into the
Northeast. Modest upper-level divergence in the form of a
strengthening 250mb jet streak aloft provides support for the
healthy shield of precipitation throughout the region, while the
air-mass remains quite cold and dry ahead of the storm (temps in
the teens,single-digit dew points). Higher SLRs of 12-16:1 are
anticipated in these areas and soil temperatures are near freezing,
which combined with the event occurring at night, will maximize
snowfall accumulation potential. The storm races off over Nova
Scotia by midday Tuesday and snowfall should taper off by Tuesday
afternoon. WPC probabilities show moderate-chance probabilities
(40-60%) for snowfall >6" in parts of northern NY (including the
Tug Hill) and in the tallest peaks of the Green and White
Mountains. These mountain ranges are favored for the heaviest
snowfall given the added help of upslope flow. Downeast Maine
sports moderate-chance probabilities for >6" of snowfall through
Tuesday morning. For those affected areas listed above, the WSSI-P
has increased its chances for Minor Impacts to moderate-to-high
chances (50-70%) from northern MI on east through northern NY and
into northern New England.

...Mid-Atlantic...
Day 2...

Residents in the Mid-Atlantic will want to monitor the forecast
closely Tuesday morning as the same tongue of moisture bringing
snow to the Northeast could bring a swath of light snow and/or
wintry mix from the central Appalachians to the New Jersey Shore.
Soils temperatures are close to freezing and light ice or snow
accumulations could make for slick travel conditions Christmas Eve
morning. WPC probabilities do show some low chance probabilities
(10-30%) for ice accumulations >0.01" in parts of the DC,
Baltimore, and Philadelphia metropolitan areas Tuesday morning.
Odds of >0.01" ice accumulations (low-to-moderate chances, or
30-50%) are greater to the west of these cities in parts of
northern MD, southeast PA, and the central Appalachians.


Mullinax




$$

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FOUS11 KWBC 232037
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
337 PM EST Mon Dec 23 2024

Valid 00Z Tue Dec 24 2024 - 00Z Fri Dec 27 2024


...The West...
Days 1-3...

An active pattern continues this week with mountain snow expected
from the West Coast mountain ranges to both the Northern and
Central Rockies. This stormy pattern in the short range is due to
three atmospheric rivers (ARs) bringing plume after plume of
Pacific moisture into the western U.S. The next AR will arrive late
this afternoon through this evening with integrated vapor transport
(IVT) values topping 1,000 kg/m/s off the Oregon coast and
moisture origins stemming out of the subtropical Pacific. This
initial round of precipitation will be primarily rain in northern
CA, western OR, and western WA as snow levels in the OR Cascades
are as high as 8,000ft and even topping 9,000ft in northern CA. The
Cascades and Blue Mountains will have the better odds of staying
mostly snow above 5,000ft.

By Tuesday morning, sharper 700-500mb height falls from the
approaching upper trough and associated surface cold front will
cause snow levels to crash through out the day. They're expected to
drop as low as 2,500 to 3,000ft at pass level in the WA Cascades
and below 6,000ft in the Sierra Nevada by Tuesday afternoon. The
Sierra Nevada have the best odds for an impactful snowfall event
with the Winter Storm Outlooks now sporting 50-70% odds of
snowfall exceeding warning criteria for elevations >7,000ft.
Periods of high elevation snow in the Great Basin and Wasatch are
likely Tuesday night but heavier accumulations (>4") will be mostly
confined to elevations at/above 8,000ft. Through 12Z Wednesday,
WPC probabilities show moderate-to- high chances (50-70%) for
snowfall >8" above 7,000ft in the Sierra Nevada, above 5,000ft in
the Cascades/Olympics/Blue Mountains, and above 8,000ft in the
Tetons.

The third and more expansive AR arrives Wednesday as a powerful
sub-960mb low tracks towards the British Columbia coast. NAEFS
shows this AR with moisture origins northwest of Hawaii that will
be directed at the Pacific Northwest, including IVT values topping
the 97.5 climatological percentile. Unlike the first pair of ARs,
this one will have a slightly colder air-mass to work with at the
onset while mean 700-300mb winds out of the WSW are better aligned
orthogonally to enhance upslope ascent into the Olympics and
Cascades. This is a recipe for heavy/wet snow in the Olympics and
Cascades Wednesday afternoon and into Wednesday night. Latest WPC
probabilities show high chances (70-80%) for >8" of snowfall for
elevations >3,000ft, and a 50-70% chances for >12" of snowfall
above 5,000ft. The WSSI-P does show 50-70% odds for Moderate
Impacts in these ranges through Thursday AM, which would include
some of the WA Cascade passes. WPC Winter Storm Outlook values
based on the latest guidance for the Washington Cascades and
Olympics as well as the ranges in eastern Oregon and Idaho have all
increased through Wednesday and Thursday to over 90% of seeing
warning criteria amounts of snow.


...Northern Plains, Great Lakes, to Northeast...
Day 1...

A storm system over the Great Lakes is causing periods of snow from
central Wisconsin through lower Michigan and beginning to spread
into northern New York this afternoon. As the storm heads east
tonight, the storm will direct its anomalous moisture and modest
low-level WAA into the Northeast. Modest upper-level divergence in
the form of a strengthening 250mb jet streak aloft provides
support for the healthy shield of precipitation throughout the
region, while the air-mass remains quite cold and dry ahead of the
storm (temps in the teens,single-digit dew points). Higher SLRs of
12-16:1 are anticipated in these areas and soil temperatures are
near freezing, which combined with the event occurring at night,
will maximize snowfall accumulation potential. The storm races off
over Nova Scotia by midday Tuesday and snowfall should taper off by
Tuesday afternoon. WPC probabilities show moderate-chance
probabilities (40-60%) for snowfall >6" in parts of northern NY
(including the Tug Hill) and in the tallest peaks of the Green and
White Mountains. These mountain ranges are favored for the heaviest
snowfall given the added help of upslope flow. Downeast Maine
sports moderate-chance probabilities for >6" of snowfall through
Tuesday morning as a secondary coastal low takes over and heads
into the Canadian Maritimes. For those affected areas listed
above, the WSSI-P has increased its chances for Minor Impacts to
moderate- to- high chances (50-70%) from northern MI on east
through northern NY and into northern New England.

...Mid-Atlantic...
Day 1...

Residents in the Mid-Atlantic will want to monitor the forecast
closely Tuesday morning as the same tongue of moisture bringing
snow to the Northeast could bring a swath of light snow and/or
wintry mix from the central Appalachians to the New Jersey Shore.
Soil temperatures are close to freezing and light ice or snow
accumulations could make for slick travel conditions Christmas Eve
morning. WPC probabilities show some low chance probabilities
(10-30%) for ice accumulations >0.01" in parts of the DC,
Baltimore, and Philadelphia metropolitan areas Tuesday morning.
Greater chances are along the Mason Dixon Line between Frederick
and Baltimore, MD through Harrisburg, PA with 20-40% chances of
.01" of freezing rain.


Wegman/Mullinax







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FOUS11 KWBC 240802
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
302 AM EST Tue Dec 24 2024

Valid 12Z Tue Dec 24 2024 - 12Z Fri Dec 27 2024


...The West...
Days 1-3...

The active weather pattern continues this morning as sharper
700-500mb height falls from the approaching upper trough and
associated surface cold front will cause snow levels to crash
through out the day. The ongoing AR will sustain an expansive IVT
above the 90th percentile according to NAEFS this morning that is
supplying ample Pacific moisture over the western third of the
CONUS. As the aforementioned upper trough continues its approach
this morning, snow levels will drop to as low as 2,500-3,000ft at
pass level in the WA Cascades and below 6,000ft in the Sierra
Nevada by Tuesday afternoon. The Sierra Nevada have the best odds
for an impactful snowfall event in large part due to the anomalous
moisture, the excellent synoptic-scale ascent ahead of the trough,
and SWrly 850-700-500mb winds approaching the 90th climatological
percentile that maximize upslope potential. WPC's Snowband
Probability Tracker snows some impressive hourly rates today that
could eclipse 3"/hr in some cases. WPC probabilities show high
chances (>70%) for snowfall totals >8" above 7,000ft with moderate
chances (40-60%) for >12" snowfall totals at/above 9,000ft in the
central and southern Sierra Nevada. Farther east, periods of high
elevation snow in the Great Basin and Wasatch are likely Tuesday
night but heavier accumulations (>4") will be mostly confined to
elevations at/above 8,000ft. Through 12Z Wednesday, WPC
probabilities show moderate-to-high chances (50-70%) for snowfall

8" above 5,000ft in the Cascades/Olympics/Blue Mountains.

The next and more expansive AR arrives Wednesday as a powerful
Pacific storm heads for the British Columbia coast. NAEFS shows
this AR, with moisture origins from the subtropical central
Pacific, will be directed at the Pacific Northwest with IVT values
topping the 99th climatological percentile. Unlike the more recent
pair of ARs, this one will have a slightly colder air-mass to work
with at the onset while mean-layer 700-300mb winds out of the WSW
are better aligned orthogonally to enhance upslope ascent into the
Olympics and Cascades. This is a recipe for heavy/wet snow in the
Olympics and Cascades Wednesday afternoon and into Wednesday night.
Latest WPC probabilities show high chances (>70%) for >8" of
snowfall for elevations >3,000ft, and 50-70% chances for >12" of
snowfall above 5,000ft. The WSSI-P does show 50-70% odds for
Moderate Impacts in these ranges through Thursday AM, which would
include some of the WA Cascade passes. The moisture associated with
this AR will spread into the Northern Rockies late Wednesday night
and into Thursday with the heaviest snowfall in the Blue Mountains

5,000ft and in the Sawtooth/Salmon River Mountains >6,000ft. WPC

probabilities depict high chances (>70%) for snowfall totals >8" at
those elevations through Thursday morning.

This unrelenting jet stream pattern looks to continue across the
Pacific Northwest on Thursday as Pacific moisture continues to be
delivered into the region ahead of the next storm system that
arrives Thursday night. The bulk of the snowfall, just like the
past couple of ARs, will remain mostly at more remote and higher
elevations from the Cascades and Olympics on south to the
Shasta/Trinity Mountains and the Sierra Nevada. This moisture
fetch will advance well inland yet again, reaching the Blue,
Sawtooth, Salmon River, Bitterroots, and Tetons Mountains. In fact,
some locally heavy snowfall may occur in the taller parks of the
Wasatch and northern Great Basin ranges. WPC probabilities show
another high chances event (>70%) for snowfall >8" above 4,000ft
in the Cascades and Olympics, above 5,000ft in the Oregon Cascades, Trinity/Shasta
in northern California, and Blue Mountains of eastern Oregon, and
above 6,000ft in the Sawtooth and Salmon River Mountains of Idaho.
Throughout the next three days (through 12Z Friday) WPC
probabilities show high chances for snowfall totals >18" in the
taller and more remote reaches of these mountain ranges. Some
impactful snowfall at pass-level and along roadways that wind
around complex terrain may cause difficult to even impossible
travel conditions through the end of the week.


...Northeast & Mid-Atlantic...
Day 1...

The storm system that brought periods of snow to the Great Lakes
continues to track east this morning while directing anomalous
moisture aloft and modest low-level WAA into the Northeast. Modest
upper-level divergence in the form of a strengthening 250mb jet
streak aloft provides support for a broad swath of precipitation
throughout the region, while the air-mass remains quite cold and
dry ahead of the storm (temps in the teens, single-digit dew
points). Higher SLRs of 12-16:1 are anticipated in these areas and
soil temperatures are near freezing, which combined with the event
occurring at night, will maximize snowfall accumulation potential.
Snow should conclude across northern and western NY by mid-morning,
followed by the rest of northern New England by midday. Downeast
Maine will witness snow the longest and thus sports high-chance
probabilities (>70%) for >6" of snowfall through Tuesday afternoon
as a secondary coastal low takes over and heads into the Canadian
Maritimes. The WSSI depicts mostly Minor impacts from central and
northern New York to eastern Maine with only portions of Downeast
Maine highlighted for seeing Moderate Impacts (hazardous driving
conditions and disruptions to daily life expected).

In the Mid-Atlantic, the same tongue of moisture bringing snow to
the Northeast will generate a swath of light snow and wintry mix
from the central Appalachians to the New Jersey Shore. Soil
temperatures are near freezing and light ice or snow accumulations
may make for slick travel conditions this morning. WPC
probabilities show some low chance probabilities (10-30%) for ice
accumulations >0.01" in parts of the DC, Baltimore, and
Philadelphia metropolitan areas this morning. Greater chances are
in portions of the central Appalachians where low-to-moderate
chances (30-50%) are present.


Mullinax










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FOUS11 KWBC 291927
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
227 PM EST Sun Dec 29 2024

Valid 00Z Mon Dec 30 2024 - 00Z Thu Jan 02 2025

...Northern Lower Michigan...
Day 1...

Storm system in the East will lift northward into Quebec tonight,
grazing eastern Lower Michigan as it heads toward Georgian Bay.
Colder air will wrap in behind the system on the west side,
changing rain to snow tonight starting at the higher elevations
(away from the lake shores) then ending Monday morning. 12Z CAM
guidance ticked up again for amounts, and a few inches are quite
likely over the northern Mitten. WPC probabilities for at least 4
inches of snow are generally <50% around Gaylord though some more
aggressive models do show some areas >6".


...The West...
Days 1-2...

Upper trough and jet will push ashore the West tonight and well
inland by tomorrow, lowering snow levels as QPF lessens. Jet will
flatten across the Great Basin but still provide broad lift on the
poleward exit region from NV/ID eastward to the Rockies. Snow
levels ahead of the front (4000-8000ft) will drop to 2000-6000ft
from north to south by tomorrow, allowing some accumulation at
lower elevations. Onshore flow off the Pacific into western WA/OR
will maintain lighter snow even as heights rise but then fall again
ahead of another (weaker) system tomorrow afternoon/evening. Much
of the snow will diminish by late Monday into Tuesday except for
over northwestern MT (Glacier NP) where additional vorticity will
push through.

For the next two days, WPC probabilities of at least 12 inches of
additional snow are high (>70%) over the higher Cascade regions in
WA/OR as well as across northern ID/northwestern MT and also
through the Tetons into the Uintas (UT) and Park Range (CO) and
into southern central WY. These probabilities lie above
~5000-6000ft to the north and 8000-9000ft or so to the south.


Pacific Northwest... Day 3...

Quasi-zonal flow below a Gulf of Alaska low will carry weaker
height falls into the PacNW coast Tuesday night with light to
moderate precip for the region into the northern Great Basin. Snow
levels are forecast to be between 2000-3000ft from north to south
across the Cascades the cold front splits off near its triple point
and skips inland. WPC probabilities for at least 6 inches of snow
are >50% above about 3000-4000ft in the WA/OR Cascades.


...North-Central Plains...
Days 1-2...

Strong trough that pushes over the Pacific Northwest today rides a
150kt zonal jet over the CO Rockies early Monday while high
pressure builds south from the lee of the Canadian Rockies.
Convergence between this high and lee cyclogenesis over KS will
support an expanding area of precipitation over eastern MT and
northeast WY tonight where it is cold enough for snow and then
across western SD through central Neb Monday where temperatures
become increasingly marginal. A general QPF swath of a quarter to a
half inch can be expected through this path, and local
enhancements from terrain (like the Black Hills) and mesoscale
banding will result in locally heavy snow. 12Z CAMS have increased
just a bit overall, and hint at potentially higher amounts than a
few inches (90th percentile >6"). WPC probabilities for at least 4
inches of snow are high (>70%) across the Black Hills, near 50%
across much of central SD and into the Pine Ridge in NE, and 10-40%
southward into northern/northeastern NE. Lower snow totals are
expected farther east with milder temperatures into Iowa.


...Northeast...
Day 3...

The low tracking over KS/MO on Monday shifts east to the Mid-
Atlantic through Tuesday before shifting up the Northeastern
Seaboard Tuesday night. It's a progressive system, but with the
surface low translating to the coast around Long Island then
strengthening as it tracks northeastward, there is a threat for
banding on the back side of the low as colder air is pulled in from
Canada. Marginal thermals should be overcome at higher elevations
over the Adirondacks and Green/White Mountains esp >1500ft. To the
west, colder air will rush in behind the system across the still
mild Great Lakes and support some lake-enhanced/effect snow
downwind of Erie/Ontario on NW flow Wednesday thru the end of this
forecast 00Z Thu. Sufficient moisture will also be present over the
central Appalachians (eastern WV into the Laurel Highlands) for
some upslope snow. WPC probabilities for at least 4 inches of snow
are moderate (30-60%) there on the west side of the system as it
retreats to Canada. Over New England, higher probabilities ~50-70%
exist over parts of the higher elevations >2500ft in the
Adirondacks, along the northern spine of the Green Mountains, and
into the White Mountains in NH into northwestern Maine where banded
snow is more likely as the low scoots across the Gulf of Maine
into eastern Maine Wednesday afternoon. Additional accumulation is
likely after 00Z Thu.


Fracasso/Jackson


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FOUS11 KWBC 300800
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
300 AM EST Mon Dec 30 2024

Valid 12Z Mon Dec 30 2024 - 12Z Thu Jan 02 2025

...Pacific Northwest...
Days 1-3...

Onshore flow and a couple impulses of upper level energy will race
across the Pacific Northwest through the day today. While snow
levels will be dropping with cold advection, the amount of moisture
over the area will also lessen with time. Snow levels will drop to
between 2,000 and 6,000 ft from north to south, which may allow
for some light accumulations in the populated valleys. The jet
stream will shift southward with time, which will also gradually
lessen the forcing for snow across much of the higher elevations of
WA/OR/ID/MT from west to east through the day. Thus, the heaviest
snow for today will be over the next few hours of this morning,
with diminishing snow over most of the mountains by tonight.
Heavier snow may persist the longest near Glacier N.P. in
northwestern MT. Quiet weather (other than some light snow at the
highest peaks near Glacier N.P. and into Wyoming) will persist
through the day Tuesday.

The next front will move ashore and into the Cascades starting
Tuesday night. It will push across the Pacific Northwest through
Wednesday night. The heaviest snow through this period will be
through the WA and OR Cascades, where some of the highest peaks of
the southern WA and OR Cascades have an over 80% chance of seeing 4
inches or more of snow through Wednesday afternoon. Through the day
Wednesday snow will move inland, dropping additional higher
elevation shows into northern Idaho and northwest Montana.
Probabilities into ID and MT are between 30-60%, with the highest
probabilities near Glacier N.P.


...North-Central Plains...
Day 1...

A 500 mb shortwave trough riding the polar jet stream will combine
with the LFQ of a 140 kt jet to cause cyclogenesis across portions
of SD and NE today. North of the low center, an expanding
precipitation shield may feature localized bands of heavier snow,
especially over south-central SD through the day today. The surface
low will track southeastward with the progressive shortwave
trough, so any one location can expect a 6-12 hour window of snow
through tonight. The heaviest snow totals are likely in the Black
Hills today where WPC probabilities of snow totals over 4 inches
are over 80% and probabilities of totals over 8 inches are over
50%. This is largely due to the localized upslope expected on the
north facing slopes which will enhance snow totals and rates.
Across the Plains of SD, probabilities of 4 inches are between
30-50% with 10-30% chances of 8 inches or more. As the low moves
into the Midwest late tonight through Wednesday, a lack of colder
air further east will make the predominant precipitation type
become rain, so the wintry threat with this low will diminish
greatly late tonight.


...Northeast...
Days 2-3...

The low tracking over KS/MO today shifts east to the Mid-Atlantic
through Tuesday before shifting up the New England coast Tuesday
night. It's a progressive system, but with the surface low
translating to the coast around Long Island then strengthening as
it tracks northeastward, there is a threat for banding on the back
side of the low as colder air is pulled in from Canada. Very warm
(for this time of year) air is in place even into interior New
England presently. By Tuesday night, the approaching precipitation
from the southwest with colder air close behind may encounter some
lingering cold air in the sheltered valleys of northern NH and
western ME. Thus, the precipitation may begin as freezing rain
before changing over to snow Wednesday. WPC probabilities for 0.01"
of ice are between 40-60%, primarily Tuesday night, for northern NH
and much of western ME.

Marginal thermals should be overcome at higher elevations over the
Adirondacks and Green/White Mountains esp >1500ft. To the west,
colder air will rush in behind the system across the still mild
Great Lakes and support some lake-enhanced/effect snow downwind of
Erie/Ontario on NW flow Wednesday thru the end of this forecast 00Z
Thu. Sufficient moisture will also be present over the central
Appalachians (eastern WV into the Laurel Highlands) for some
upslope snow. WPC probabilities for at least 4 inches of snow are
moderate (30-60%) there on the west side of the system as it
retreats to Canada. Over New England, higher probabilities ~50-70%
exist over parts of the higher elevations >2500ft in the
Adirondacks, along the northern spine of the Green Mountains, and
into the White Mountains in NH into northwestern Maine where banded
snow is more likely as the low scoots across the Gulf of Maine
into eastern Maine Wednesday afternoon. Snow will continue into
interior New England through Wednesday night. Meanwhile lake-effect
and upslope snow will keep light snow going through Wednesday night
from western New York and northeast Ohio through the mountains of
western Pennsylvania and West Virginia.

Wegman

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FOUS11 KWBC 310905
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
405 AM EST Tue Dec 31 2024

Valid 12Z Tue Dec 31 2024 - 12Z Fri Jan 03 2025

...Pacific Northwest to Northern Rockies...
Days 1-3...

Lull today over the Northwest ends this evening as a plume of
Pacific moisture streams in ahead of a trough emanating from a
Gulf of Alaska low. A vort lobe from this trough ejects ESE over WA
tonight and Wyoming Wednesday night. The front with this wave
stalls near the OR/WA border tonight before slowly lifting north
through Thursday as the intensity of moisture advection increases.
Snow levels of 2000 to 3000 ft linger over the Cascades into
Wednesday before rising steadily south of the front with levels in
southern OR reaching 7000ft while they remain 2000ft in the North
WA Cascades (though rates generally remain light north of the
front). Day 1.5 snow probs for >8" are 50-90% in the OR Cascades
and 30-50% in the southern WA Cascades. This moisture shifts inland
with Day 2 snow probs for >8" 30-60% over the Sawtooths and Tetons
south along the WY/ID border. A powerful low which will be
directing the atmospheric river approaches the OR Coast Thursday
night. Snow levels of 6000-8000ft persist over OR with lower values
in WA allowing some Day 3 snow probs for >8" around 30% in the
North WA Cascades. Sufficient cold air pools in the Columbia Basin
by Thursday night to allow a threat of ice accretion in the lower
Basin into the Columbia Gorge Thursday night.


...Great Lakes, Central Appalachians, Northeast...
Days 1-3...

The surface low currently crossing St. Louis is presenting
deformation banding over IA and northern IL, but
uncharacteristically for a late December night, there is
insufficient cold air for significant snowfall. However, cold
conveyor flow from the east and further development should allow
for some decent snow bands to develop later this morning over
southern MI and far northern IN. Tonight expect LES bands to form
off Lakes Superior and Michigan that persist into Thursday. Day 2.5
snow probs are high near Whitefish Bay.
A coastal low develops at the triple- point over the central Mid-
Atlantic this evening and quickly becoming the dominant low. This
low tracks up the New England coast through Wednesday with dynamic
cooling driven snow over terrain of northern New England late
tonight into Thursday before lake enhanced snow develops in to lake
effect snow bands off the still ice free Lakes Erie and Ontario.
Day 2.5 snow probs for >8" are 50-80% for the Tug Hill and northern
Adirondacks as well as the length of the Greens in NT and the
Presidential Range in NH.
Upslope flow into the central Appalachians turns into snow late
tonight that continues through Wednesday. Day 1.5 snow probs for

6" are 40-80% from the PA Laurels through the Potomac and

Allegheny Highlands west from the Allegheny Front. The progression
of the low slows over Atlantic Canada Wednesday night through
Thursday night which allows LES to persist. Day 2 snow probs are
50-80% from Erie through southern Buffalo and around 80% on the Tug
Hill. The focus shifts a bit north for Day 3 with snow probs for
an additional >8" over the Tug Hill around 60%.


Jackson



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FOUS11 KWBC 311958
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
258 PM EST Tue Dec 31 2024

Valid 12Z Tue Dec 31 2024 - 12Z Fri Jan 03 2025

...Pacific Northwest to Northern Rockies...
Days 1-3...

A 500mb vorticity maximum shearing off from an upper low over the
North Pacific will escort a slug of moisture into the Pacific
Northwest tonight. This elongated area of upper-level divergence
aloft paired with a couple frontal systems that will provide a
source of lift at low levels as well. Snow levels as low as 2,000ft
are expected initially over western Washington, but as the warm
front approaches the northern California and southern Oregon coast
Wednesday evening, snow levels in the southern Oregon Cascades
will rise to as high as 7,000ft by Wednesday night. This milder
air-mass remains in place ahead of the next storm system that
arrives on Thursday with rain being the primary precipitation type
south of the Columbia River. The Cascades and Olympics will be the
mountain ranges likely to remain mostly snow above 3,000ft with
locally heavier totals above 4,000ft. Meanwhile, sub-freezing wet-
bulb temperatures may become more prominent in the Columbia Basin
and along the leeward slopes of the Cascades Thursday afternoon.
This could result in some minor ice accumulations in these areas
through Thursday night. The active Pacific pattern continues into
Friday as another plume of moisture surges into northern California
and the Oregon Cascades. Steeper 700-500mb height falls associated
with this system means plummeting snow levels to where parts of the Trinity/Shasta and Oregon Cascades above 5,000ft could see locally
heavy snowfall. In total over the next three days, the Oregon
Cascades have high chances (>70%) for snowfall totals >8" through
Wednesday afternoon. After the brief lull and rising snow levels
Wednesday night, the Thursday system then gives the Washington
Cascades moderate-to-high chances (50-70%) for >6" of snowfall at
elevations above 4,000ft.

Farther east, the Northern Rockies best chances for accumulating
snowfall arrives New Year's Day and into early Thursday (Jan. 2)
when the first 500mb trough pushing through the Pacific Northwest
Wednesday morning provides modest upper level divergence aloft and
directs the same plume of Pacific moisture towards the higher
elevations of the northern Great Basin, the central Idaho ranges,
and into both the Bear River and Teton Ranges. The two latter
ranges above 8,000ft sport high chance probabilities (>70%) for
snowfall >8" through Thursday morning. As far east as the >9,000ft
peaks of the northern Colorado Rockies have similar high chance
probabilities for >8" of snowfall between Wednesday morning and
Thursday morning.

...Great Lakes, Central Appalachians, Northeast...
Days 1-3...

A dynamic and complicated storm evolution will make for a
synoptic-scale snowfall event through Wednesday evening, then
transition into a lake-effect snow (LES) event Wednesday night and
through the remainder of the week. Focusing on the synoptic-scale
event, an occluded low tracking through northern Ohio is rather
anomalous with NAEFS showing MSLP values below thew 1st
climatological percentile over the upper Ohio Valley. The occluded
front will spawn a new wave of low pressure along the triple point
located along the New Jersey coast tonight while, at the same time,
the 250-500mb trough aloft maintains a negative tilt over
Pennsylvania. The excellent divergence atop the atmosphere over
the Northeast will lead to a steady deepening of the surface
cyclone throughout New Year's Day. As the primary occluded low
weakens, it will effectively turn into a TROWAL over the eastern
Great Lakes late tonight and into New Year's morning. This TROWAL
will work in tandem with accelerating low-level NWrly flow over
Lakes Huron and Erie to support lake-enhanced snowfall rates over
northeast Ohio, northwest Pennsylvania, and western New York. WPC's
Snowband Probability Tracker suggested that a couple HREF members
showed >1"/hr snowfall rates in these areas Wednesday morning.

Farther east, there is growing consensus on an impressive 850-700mb
FGEN signal over northern New England that is paired with a
southeasterly IVT that tops the 90th climatological percentile.
With no shortage of moisture and impressive mesoscale forcing,
strong dynamic cooling over the Whites and northern Maine will
result in heavy-banded snowfall Wednesday morning and into
Wednesday afternoon. By Wednesday evening, the steadily deepening
cyclone over the Gulf of Maine will wrap Atlantic moisture around
the western flank of the low and lead to a deformation zone along
the Maine/Canada border while wrap around moisture and upslope
ascent via NWrly flow strikes the White Mountains. Moisture
associated with the TROWAL will also support prolonged lift, with
assistance from upslope enhancement, in the Adirondacks, Greens,
and as far south as the Berkshires Wednesday night into Thursday
morning. Cyclonic flow will persist through Thursday night and into
Friday as the cyclone has effectively no where to go over Quebec
thanks to a strong -NAO block over Greenland and the Davis Strait.

The prolonged cyclonic flow will be responsible for a multi-day
LES setup as far west as the Michigan U.P. and as far east as the
Tug Hill and Finger Lakes of central New York. The heaviest
snowfall via LES bands begins in the Michigan U.P. starting
Wednesday morning, while LES really kicks in late Wednesday night
or Thursday morning for the eastern Great Lakes. WPC probabilities
show moderate-to-high chances (50-70%) for snowfall >18" in parts
of the Tug Hill, along the Chautauqua Ridge, and in some select
parts of the eastern Michigan U.P.. Through the three-day span, the
WSSI shows Major Impacts along the Chautauqua Ridge (including
some portions of I-90) and the Tug Hill. These areas can expect
considerable disruptions to daily life with dangerous to near
impossible travel the most notable impact. Moderate Impacts
(hazardous travel conditions) are possible in areas just east of
Cleveland and within the Syracuse metro area. This same cyclonic
flow regime will be the catalyst for heavy mountain snow in the
central Appalachians between early Wednesday morning and Thursday
morning. WPC probabilities show moderate-to-high chances (50-70%)
for snowfall totals >6" above 2,500ft in the Laurel Highlands on
south into the east-central West Virginia Appalachians.

...Midwest...
Day 2...

The sheared off 500mb vorticity max that tracked through the
Pacific Northwest on Wednesday will head for the Central Plains
Wednesday night. Surface-850mb frontogenesis over Nebraska will
help to spawn low pressure along the front and track east towards
the Missouri River late Wednesday night and into Thursday morning.
The storm system will be a small and compact one, but the presence
of strong 850mb FGEN and an air-mass plenty cold enough to support
snow could generate a narrow band of heavy snow from eastern
Nebraska to central Iowa. As 850mb WAA wains Wednesday afternoon,
the mesoscale lift driving the snow across parts of eastern
Nebraska and central Iowa will gradually taper off as it moves east
into Illinois and Indiana. WPC probabilities show low chances
(10-30%) for >4" amounts in west-central Iowa, but this event is
more likely to support a general 2-4" event with moderate
probabilities (40-60%) for >2" of snow from northeast Nebraska to
south-central Iowa Thursday morning.


Mullinax






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FOUS11 KWBC 010929
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
429 AM EST Wed Jan 1 2025

Valid 12Z Wed Jan 01 2025 - 12Z Sat Jan 04 2025

...The West...
Days 1-3...

A prolonged atmospheric river (AR) event continues to provide
moisture to the northwestern states through Friday night. A potent
trough crosses the Pacific Northwest coast on Friday which will
define the end of this AR, though an active winter pattern is
expected to continue over the Northwest through the weekend.

Rising snow levels in the core of the AR today into the OR/CA
border cause snow levels on the Klamath and southern Cascades to
rise above 7000ft.

...North-Central Rockies...
Days 1-2...
A shortwave impulse on the leading edge of the AR pushes ESE from
Washington state this morning and crosses Wyoming this evening.
Sufficient moisture, baroclinicity from existing stalled fronts,
and topographic lift will bring snow to the north- central Rockies
today into Thursday. Lee cyclogenesis aids develop over north-
central CO terrain that persists through Thursday morning. Day 1
snow probs for >8" is 40-80% for much of the central/southern ID
terrain, from the Tetons through the Wasatch, and northern CO
ranges that extend into southern WY.

Pacific Northwest into California...
Days 2-3...
Ridging behind the impulse causes further height rises as massive
AR moisture continues to stream in. A persistent frontal boundary
near the WA/OR border allows snow levels over the WA Cascades to
remain 4000ft or less through Thursday. Day 2 snow probs for >6"
are 20-40% over the WA Cascades. Meanwhile, sub-freezing wet-bulb
temps in the Columbia Basin and possibly Gorge Thursday afternoon
could result in some minor ice accumulations through Thursday
night.

A potent trough south of a mid-level low that tracks into western
WA on Friday provides a focus for lift with the robust moisture in
the AR for the Cascades through the northern Sierra Nevada while
height falls lower the snow level before the enhanced precip
diminishes Friday night. Snow levels of 7000-8000ft from central OR
through northern CA late Thursday night drop below 6000ft Friday
afternoon under the trough with snow levels of 4000-5000ft
persisting over WA. Day 3 snow probs for >8" are generally 20-40%
from the northern Sierra through southern WA terrain, while values
are 30-70% in the northern WA Cascades and in terrain of far
northern ID into MT.


...Great Lakes and Northeast...
Days 1-3...

Developing low over NY will track north over Maine through
tonight the drift farther north over the Canadian Maritimes into
Friday night causing snow over Northeast terrain today with lake
enhanced snow this evening becoming lake effect snow over the
Great Lakes that persist in earnest through Friday night. Snow this
morning is focused on the White Mtns above 2000ft snow levels with
heavy rates in the higher terrain and far interior Maine this
afternoon. Heavy lake enhanced snow develops off Lake Erie this
morning, shifting up off Lake Ontario and over the Adirondacks and
the Green Mtns this evening. Day 1 snow probs for >6" are highest
east of Erie, Ontario and the northern Adirondacks. Rates should
exceed 1"/hr for a few hours in each of these areas raising the
threat. Single banded LES persists from Lake Ontario through Friday
night with the WNW flow maximizing upstream moisture fetch with
Lake Superior, northern Lake Huron and the Georgian Bay (all of
which are nearly open water) in the stream lines for the Tug Hill.
Day 2 snow probs are 90% for >8" for the Tug Hill and around 60%
for Day 3.

Meanwhile, general multi- band LES is expected off Superior and MI
all three days on WNW flow. The heaviest snow is through Thursday
in the U.P. where preferred snow belts in WNW flow have >50% probs
for >6".



...Midwest through Central Appalachians...
Days 1-3...

The shortwave impulse crossing ESE over WY this evening and
promoting lee cyclogenesis tonight over CO will track over the
Central Plains late tonight into Thursday. Surface-850mb
frontogenesis over northeast Nebraska will help to draw an
inverted trough from KS which will track to KY by Thursday evening.
low pressure along the front and track east towards the Missouri
This wave is compact, but the presence of strong 850mb FGEN and air
cold enough for snow will generate narrow bands of moderate to
locally heavy snow from northeastern Nebraska through Iowa and
possibly IL/IN/OH (though the wave weakens). Day 1.5 snow probs
for >2" are 30-70% over northeast Neb and much of IA with values
generally 10% or less over northern IL/IN into OH.
A northern stream impulse interacts with this wave on Friday just
before it reaches the central/WV Appalachians which will enhance
lift along with topography and Great Lakes moisture to produce
enhanced upslope snow from the PA Laurels through the Allegheny
Highlands of WV. Day 3 snow probs for >6" are 50-80% from around Mt
Davis down the Allegheny Front through the Allegheny Highlands.


Jackson



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FOUS11 KWBC 011933
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
233 PM EST Wed Jan 1 2025

Valid 12Z Wed Jan 01 2025 - 12Z Sat Jan 04 2025

...The West...
Days 1-3...

Persistent troughing across the Pacific will maintain broad SW flow
into a modest ridge along the coast until Saturday when a
pronounced closed low digs eastward towards OR forcing a trough to
move inland to the Great Basin by the end of the forecast period.

Periodic impulses shedding through the flow will move into a region
of greater mid-level divergence D1 and the first half of D2,
leading to broad ascent and increasing precipitation coverage. For
D1, this will be most pronounced along a low-level warm
front/baroclinic gradient lifting east/northeast from OR into the
Central and Northern Rockies tonight and Thursday. This will
result in a swath of precipitation, primarily as snow, but with
snow levels rising to as high as 6000 ft in southern OR/northern
Great Basin, with a sharp gradient to less than 1000 ft into the
Northern Rockies and Columbia Basin. The heaviest snow D1 is
expected in the Central Rockies from the Tetons through the Park
Range where higher SLR and strong upslope flow will enhance
accumulations. Here, WPC probabilities are above 70% for 8+ inches
of snow in the higher terrain, with 1-2 feet of snow possible.

A brief respite in heavier snowfall is likely the first half of D2
before more substantial moisture and ascent spread onshore
late Friday and through Saturday. This more enhanced
forcing/moisture is associated with the closed low moving onshore
and causing the entire trough axis to pivot eastward. Impressive
lift will be aided by the LFQ of a modest, but well placed, upper
jet streak, and there is likely to be strong deep layer lift
expanding across much of the Pacific and Interior Northwest Friday
night, shifting as far as the Northern High Plains by Saturday
night. The most impressive ascent will again be collocated with a
secondary warm front lifting northward from OR to Canada on Friday,
leading to enhanced WAA/isentropic ascent within the broad synoptic
lift regime. While overall omega is modest, a swath of moderate to
heavy snow is likely from the mountainous areas of northern CA
through OR, WA, and into the Northern Rockies, with snow levels
generally rising to 4000-6000 ft. This is reflected by WPC
probabilities which suggest a high risk (70-90%) for more than 6
inches of snow in the Cascades and Northern Rockies, with more
general onshore flow leading to upslope snow across the typical
terrain features D3 from the Sierra, Cascades, and into the
Northern Rockies again.

Additionally, the warm advection overrunning cold high pressure
centered over Saskatchewan is progged to leave low-level cold
easterly flow from the Columbia basin into the Columbia Gorge.
Although eventually this will be scoured out, the setup appears
favorable for a prolonged period of moderate freezing rain in the
eastern foothills of the Cascades and into the Columbia
Gorge/Basin. The heaviest icing is expected along the Hood River
Valley and into the Gorge which will take a longer time to scour
out the cold air, and this is reflected by WPC probabilities that
reach as high as 30% for 0.1" of ice accretion.


...Great Lakes and Northeast...
Days 1-3...

Surface low pressure will be moving across eastern Maine and into
the Canadian maritimes to begin to the period, ushering in a long
duration lake effect snow (LES) event into the weekend. This
surface low will be pushed northeast in response to a potent
vorticity maxima rotating through the negatively tilted trough, and
although this low could deepen rapidly in response to favorable
overlap of PVA, height falls, and LFQ diffluence beneath a zonally
aligned jet streak to the south, the best moisture transport will
shift away from Maine early D1. Still, some WAA snowfall will
blanket eastern and northern Maine through Thursday morning with
modest accumulations.

The more significant snow associated with this system will occur in
its wake as N/NW flow develops across the Great Lakes and
Northeast, leading to increased coverage of both LES and upslope
flow into the Upstate NY/New England terrain. This will be in
response to a persistent but amplifying mid-level trough expanding
across the east, with weak vorticity lobes swinging through the
flow to help sharpen the trough and enhanced CAA. This will lead to
enhanced upslope flow and heavy snow, especially in the
Adirondacks, Greens, and mountains of NW Maine through D1 and into
D2, with a deep DGZ and impressive lift leading to WPC
probabilities that feature a high risk (>70% chance) for 8+ inches
of snow in the NW Adirondacks and northern Greens.

More substantial accumulations are likely, however, within the LES,
especially late D1 and through D2 as lake-induced instability
climbs above 500 J/kg. The pattern will support heavy LES in the
favored W/NW snow belts from the U.P., across the L.P., and then
downstream to east of Lakes Erie and Ontario, with a favorable
upstream moisture connection from Lakes Superior and Huron
enhancing LES off of Lake Ontario. LES is expected to be most
widespread and heavy D1 and D2, when 1-2"/hr snows, possibly
greater at times, cause WPC probabilities to be high (>70%) for 6+
inches downstream of most Lakes, and reach above 90% for 8+ inches
east of Lake Ontario D2. Total snowfall of multiple feet is likely,
with the highest along the Tug Hill Plateau. LES will continue
through D3 with just slightly lesser intensity and coverage as
reflected by WPC probabilities for an additional 6+ inches being
above 70% in just narrow channels near the Chautauqua Ridge and the
Tug Hill Plateau.


...Midwest through Central Appalachians...
Days 1-3...

An inverted surface trough sandwiched between a retreating high
pressure over the Southeast and an incoming strong Canadian high
pressure from Alberta will become a focus for a developing wave of
low pressure and an associated stripe of snowfall from west to
east. The sharpening of this trough will initially be in response
to a subtle vorticity lobe swinging through the broad trough across
the east, ejecting from the High Plains into the Upper Midwest
Thursday. Immediately following this impulse, a potent jet streak
will drop southeast into the Northern/Central Plains while
amplifying to 150 kts, providing favorable overlap for deep layer
ascent. This will stem cyclogenesis along the inverted trough, with
increasing downstream warm/moist advection surging moisture along
the 280-285K isentropic surfaces into the Upper Midwest. This WAA
will help deepen the DGZ to the east (SREF probabilities for 100mb
of DGZ depth climbing to 10-30% from the Upper Midwest into the OH
VLY), while 925-850mb fgen increases, although only modestly.

This will result in a stripe of moderate snow from Iowa through the
Ohio Valley, where at least brief snowfall rates to 1"/hr are
possible, leading to WPC probabilities for more than 2 inches that
reach 70-90% in central IA, with locally up to 4" possible.

Farther downstream, as the low coalesces to be more pronounced and
has stronger frontal features accompanying it, a period of strong
upslope flow will occur in its wake across the Central
Appalachians. The DGZ is shallow and thin, but pronounced lift into
it will result in a period of heavy snow, especially late Friday
into Saturday, from the Laurel Highlands through southern WV.
During this time, moisture will be enhanced as well from upstream
Great Lakes connection, enhancing the potential for heavy
accumulations, and WPC probabilities D2 are high (>70%) and
moderate (30-50%) D3 for 4+ inches of snow, leading to local maxima
around 12 inches by the end of the forecast period.

Finally, although accumulations are expected to be minimal, at
worst, some light snow may crest the Appalachians and move across
the Mid-Atlantic states Friday afternoon bringing at least
conversational snow to the areas around Washington, D.C.


...Northern High Plains...
Day 3...

A stripe of heavy snowfall is becoming more likely Saturday from
central MT through western SD. This snowfall will be associated
with a slowly advancing warm front topped by increasing divergence
downstream of a potent shortwave digging into the Four Corners
region. While jet dynamics are weak during this time, the
downstream moist advection on impressive 280-290K isentropic ascent
should expand precipitation in a NW to SE fashion late Friday night
and through Saturday. Robust 850-700mb fgen into this moistening
column will help intensify ascent resulting in heavier snow rates,
but at this time the intensity of this snow is still uncertain due
to modest DGZ depth probabilities and a lack of ideal overlap
between the best ascent and the DGZ. Still, a band of moderate to
heavy snow is becoming more likely, and WPC probabilities have
increased to 30-50% for 4+ inches across north-central MT.
Additional increases are possible with later model runs.


Weiss



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FOUS11 KWBC 012007
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
307 PM EST Wed Jan 1 2025

Valid 00Z Thu Jan 02 2025 - 00Z Sun Jan 05 2025

...The West...
Days 1-3...

Persistent troughing across the Pacific will maintain broad SW flow
into a modest ridge along the coast until Saturday when a
pronounced closed low digs eastward towards OR forcing a trough to
move inland to the Great Basin by the end of the forecast period.

Periodic impulses shedding through the flow will move into a region
of greater mid-level divergence D1 and the first half of D2,
leading to broad ascent and increasing precipitation coverage. For
D1, this will be most pronounced along a low-level warm
front/baroclinic gradient lifting east/northeast from OR into the
Central and Northern Rockies tonight and Thursday. This will
result in a swath of precipitation, primarily as snow, but with
snow levels rising to as high as 6000 ft in southern OR/northern
Great Basin, with a sharp gradient to less than 1000 ft into the
Northern Rockies and Columbia Basin. The heaviest snow D1 is
expected in the Central Rockies from the Tetons through the Park
Range where higher SLR and strong upslope flow will enhance
accumulations. Here, WPC probabilities are above 70% for 8+ inches
of snow in the higher terrain, with 1-2 feet of snow possible.

A brief respite in heavier snowfall is likely the first half of D2
before more substantial moisture and ascent spread onshore
late Friday and through Saturday. This more enhanced
forcing/moisture is associated with the closed low moving onshore
and causing the entire trough axis to pivot eastward. Impressive
lift will be aided by the LFQ of a modest, but well placed, upper
jet streak, and there is likely to be strong deep layer lift
expanding across much of the Pacific and Interior Northwest Friday
night, shifting as far as the Northern High Plains by Saturday
night. The most impressive ascent will again be collocated with a
secondary warm front lifting northward from OR to Canada on Friday,
leading to enhanced WAA/isentropic ascent within the broad synoptic
lift regime. While overall omega is modest, a swath of moderate to
heavy snow is likely from the mountainous areas of northern CA
through OR, WA, and into the Northern Rockies, with snow levels
generally rising to 4000-6000 ft. This is reflected by WPC
probabilities which suggest a high risk (70-90%) for more than 6
inches of snow in the Cascades and Northern Rockies, with more
general onshore flow leading to upslope snow across the typical
terrain features D3 from the Sierra, Cascades, and into the
Northern Rockies again.

Additionally, the warm advection overrunning cold high pressure
centered over Saskatchewan is progged to leave low-level cold
easterly flow from the Columbia basin into the Columbia Gorge.
Although eventually this will be scoured out, the setup appears
favorable for a prolonged period of moderate freezing rain in the
eastern foothills of the Cascades and into the Columbia
Gorge/Basin. The heaviest icing is expected along the Hood River
Valley and into the Gorge which will take a longer time to scour
out the cold air, and this is reflected by WPC probabilities that
reach as high as 30% for 0.1" of ice accretion.


...Great Lakes and Northeast...
Days 1-3...

Surface low pressure will be moving across eastern Maine and into
the Canadian maritimes to begin to the period, ushering in a long
duration lake effect snow (LES) event into the weekend. This
surface low will be pushed northeast in response to a potent
vorticity maxima rotating through the negatively tilted trough, and
although this low could deepen rapidly in response to favorable
overlap of PVA, height falls, and LFQ diffluence beneath a zonally
aligned jet streak to the south, the best moisture transport will
shift away from Maine early D1. Still, some WAA snowfall will
blanket eastern and northern Maine through Thursday morning with
modest accumulations.

The more significant snow associated with this system will occur in
its wake as N/NW flow develops across the Great Lakes and
Northeast, leading to increased coverage of both LES and upslope
flow into the Upstate NY/New England terrain. This will be in
response to a persistent but amplifying mid-level trough expanding
across the east, with weak vorticity lobes swinging through the
flow to help sharpen the trough and enhanced CAA. This will lead to
enhanced upslope flow and heavy snow, especially in the
Adirondacks, Greens, and mountains of NW Maine through D1 and into
D2, with a deep DGZ and impressive lift leading to WPC
probabilities that feature a high risk (>70% chance) for 8+ inches
of snow in the NW Adirondacks and northern Greens.

More substantial accumulations are likely, however, within the LES,
especially late D1 and through D2 as lake-induced instability
climbs above 500 J/kg. The pattern will support heavy LES in the
favored W/NW snow belts from the U.P., across the L.P., and then
downstream to east of Lakes Erie and Ontario, with a favorable
upstream moisture connection from Lakes Superior and Huron
enhancing LES off of Lake Ontario. LES is expected to be most
widespread and heavy D1 and D2, when 1-2"/hr snows, possibly
greater at times, cause WPC probabilities to be high (>70%) for 6+
inches downstream of most Lakes, and reach above 90% for 8+ inches
east of Lake Ontario D2. Total snowfall of multiple feet is likely,
with the highest along the Tug Hill Plateau. LES will continue
through D3 with just slightly lesser intensity and coverage as
reflected by WPC probabilities for an additional 6+ inches being
above 70% in just narrow channels near the Chautauqua Ridge and the
Tug Hill Plateau.


...Midwest through Central Appalachians...
Days 1-3...

An inverted surface trough sandwiched between a retreating high
pressure over the Southeast and an incoming strong Canadian high
pressure from Alberta will become a focus for a developing wave of
low pressure and an associated stripe of snowfall from west to
east. The sharpening of this trough will initially be in response
to a subtle vorticity lobe swinging through the broad trough across
the east, ejecting from the High Plains into the Upper Midwest
Thursday. Immediately following this impulse, a potent jet streak
will drop southeast into the Northern/Central Plains while
amplifying to 150 kts, providing favorable overlap for deep layer
ascent. This will stem cyclogenesis along the inverted trough, with
increasing downstream warm/moist advection surging moisture along
the 280-285K isentropic surfaces into the Upper Midwest. This WAA
will help deepen the DGZ to the east (SREF probabilities for 100mb
of DGZ depth climbing to 10-30% from the Upper Midwest into the OH
VLY), while 925-850mb fgen increases, although only modestly.

This will result in a stripe of moderate snow from Iowa through the
Ohio Valley, where at least brief snowfall rates to 1"/hr are
possible, leading to WPC probabilities for more than 2 inches that
reach 70-90% in central IA, with locally up to 4" possible.

Farther downstream, as the low coalesces to be more pronounced and
has stronger frontal features accompanying it, a period of strong
upslope flow will occur in its wake across the Central
Appalachians. The DGZ is shallow and thin, but pronounced lift into
it will result in a period of heavy snow, especially late Friday
into Saturday, from the Laurel Highlands through southern WV.
During this time, moisture will be enhanced as well from upstream
Great Lakes connection, enhancing the potential for heavy
accumulations, and WPC probabilities D2 are high (>70%) and
moderate (30-50%) D3 for 4+ inches of snow, leading to local maxima
around 12 inches by the end of the forecast period.

Finally, although accumulations are expected to be minimal, at
worst, some light snow may crest the Appalachians and move across
the Mid-Atlantic states Friday afternoon bringing at least
conversational snow to the areas around Washington, D.C.


...Northern High Plains...
Day 3...

A stripe of heavy snowfall is becoming more likely Saturday from
central MT through western SD. This snowfall will be associated
with a slowly advancing warm front topped by increasing divergence
downstream of a potent shortwave digging into the Four Corners
region. While jet dynamics are weak during this time, the
downstream moist advection on impressive 280-290K isentropic ascent
should expand precipitation in a NW to SE fashion late Friday night
and through Saturday. Robust 850-700mb fgen into this moistening
column will help intensify ascent resulting in heavier snow rates,
but at this time the intensity of this snow is still uncertain due
to modest DGZ depth probabilities and a lack of ideal overlap
between the best ascent and the DGZ. Still, a band of moderate to
heavy snow is becoming more likely, and WPC probabilities have
increased to 30-50% for 4+ inches across north-central MT.
Additional increases are possible with later model runs.


...Central Plains through the Mid-Atlantic...
A significant winter storm is becoming more likely Saturday through
Monday, with the onset of light freezing or frozen precipitation
beginning Saturday evening across Kansas. At this, time, WPC
probabilities for significant snow or ice are minimal through the
end of D3, but heavy snow and significant icing is expected into D4
and beyond. For this system, WPC has initiated Key Messages which
are linked below, and more information can be found in the WPC
extended range discussion as well.


Weiss


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FOUS11 KWBC 012009
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
309 PM EST Wed Jan 1 2025

Valid 00Z Thu Jan 02 2025 - 00Z Sun Jan 05 2025

...The West...=20
Days 1-3...

Persistent troughing across the Pacific will maintain broad SW flow
into a modest ridge along the coast until Saturday when a
pronounced closed low digs eastward towards OR forcing a trough to
move inland to the Great Basin by the end of the forecast period.=20

Periodic impulses shedding through the flow will move into a region
of greater mid-level divergence D1 and the first half of D2,
leading to broad ascent and increasing precipitation coverage. For
D1, this will be most pronounced along a low-level warm
front/baroclinic gradient lifting east/northeast from OR into the=20
Central and Northern Rockies tonight and Thursday. This will=20
result in a swath of precipitation, primarily as snow, but with=20
snow levels rising to as high as 6000 ft in southern OR/northern=20
Great Basin, with a sharp gradient to less than 1000 ft into the=20
Northern Rockies and Columbia Basin. The heaviest snow D1 is
expected in the Central Rockies from the Tetons through the Park
Range where higher SLR and strong upslope flow will enhance
accumulations. Here, WPC probabilities are above 70% for 8+ inches
of snow in the higher terrain, with 1-2 feet of snow possible.

A brief respite in heavier snowfall is likely the first half of D2
before more substantial moisture and ascent spread onshore
late Friday and through Saturday. This more enhanced
forcing/moisture is associated with the closed low moving onshore
and causing the entire trough axis to pivot eastward. Impressive
lift will be aided by the LFQ of a modest, but well placed, upper
jet streak, and there is likely to be strong deep layer lift
expanding across much of the Pacific and Interior Northwest Friday
night, shifting as far as the Northern High Plains by Saturday
night. The most impressive ascent will again be collocated with a
secondary warm front lifting northward from OR to Canada on Friday,
leading to enhanced WAA/isentropic ascent within the broad synoptic
lift regime. While overall omega is modest, a swath of moderate to
heavy snow is likely from the mountainous areas of northern CA
through OR, WA, and into the Northern Rockies, with snow levels
generally rising to 4000-6000 ft. This is reflected by WPC
probabilities which suggest a high risk (70-90%) for more than 6
inches of snow in the Cascades and Northern Rockies, with more=20
general onshore flow leading to upslope snow across the typical=20
terrain features D3 from the Sierra, Cascades, and into the=20
Northern Rockies again.

Additionally, the warm advection overrunning cold high pressure
centered over Saskatchewan is progged to leave low-level cold
easterly flow from the Columbia basin into the Columbia Gorge.
Although eventually this will be scoured out, the setup appears
favorable for a prolonged period of moderate freezing rain in the
eastern foothills of the Cascades and into the Columbia
Gorge/Basin. The heaviest icing is expected along the Hood River
Valley and into the Gorge which will take a longer time to scour=20
out the cold air, and this is reflected by WPC probabilities that
reach as high as 30% for 0.1" of ice accretion.


...Great Lakes and Northeast...=20
Days 1-3...

Surface low pressure will be moving across eastern Maine and into
the Canadian maritimes to begin to the period, ushering in a long
duration lake effect snow (LES) event into the weekend. This
surface low will be pushed northeast in response to a potent
vorticity maxima rotating through the negatively tilted trough, and
although this low could deepen rapidly in response to favorable
overlap of PVA, height falls, and LFQ diffluence beneath a zonally
aligned jet streak to the south, the best moisture transport will
shift away from Maine early D1. Still, some WAA snowfall will
blanket eastern and northern Maine through Thursday morning with
modest accumulations.=20

The more significant snow associated with this system will occur in
its wake as N/NW flow develops across the Great Lakes and
Northeast, leading to increased coverage of both LES and upslope
flow into the Upstate NY/New England terrain. This will be in
response to a persistent but amplifying mid-level trough expanding
across the east, with weak vorticity lobes swinging through the
flow to help sharpen the trough and enhanced CAA. This will lead to
enhanced upslope flow and heavy snow, especially in the
Adirondacks, Greens, and mountains of NW Maine through D1 and into
D2, with a deep DGZ and impressive lift leading to WPC
probabilities that feature a high risk (>70% chance) for 8+ inches
of snow in the NW Adirondacks and northern Greens.

More substantial accumulations are likely, however, within the LES,
especially late D1 and through D2 as lake-induced instability
climbs above 500 J/kg. The pattern will support heavy LES in the
favored W/NW snow belts from the U.P., across the L.P., and then
downstream to east of Lakes Erie and Ontario, with a favorable
upstream moisture connection from Lakes Superior and Huron
enhancing LES off of Lake Ontario. LES is expected to be most
widespread and heavy D1 and D2, when 1-2"/hr snows, possibly
greater at times, cause WPC probabilities to be high (>70%) for 6+
inches downstream of most Lakes, and reach above 90% for 8+ inches
east of Lake Ontario D2. Total snowfall of multiple feet is likely,
with the highest along the Tug Hill Plateau. LES will continue=20
through D3 with just slightly lesser intensity and coverage as=20
reflected by WPC probabilities for an additional 6+ inches being
above 70% in just narrow channels near the Chautauqua Ridge and the
Tug Hill Plateau.


...Midwest through Central Appalachians...=20
Days 1-3...

An inverted surface trough sandwiched between a retreating high
pressure over the Southeast and an incoming strong Canadian high
pressure from Alberta will become a focus for a developing wave of
low pressure and an associated stripe of snowfall from west to
east. The sharpening of this trough will initially be in response
to a subtle vorticity lobe swinging through the broad trough across
the east, ejecting from the High Plains into the Upper Midwest
Thursday. Immediately following this impulse, a potent jet streak
will drop southeast into the Northern/Central Plains while
amplifying to 150 kts, providing favorable overlap for deep layer
ascent. This will stem cyclogenesis along the inverted trough, with
increasing downstream warm/moist advection surging moisture along
the 280-285K isentropic surfaces into the Upper Midwest. This WAA
will help deepen the DGZ to the east (SREF probabilities for 100mb
of DGZ depth climbing to 10-30% from the Upper Midwest into the OH
VLY), while 925-850mb fgen increases, although only modestly.=20

This will result in a stripe of moderate snow from Iowa through the
Ohio Valley, where at least brief snowfall rates to 1"/hr are
possible, leading to WPC probabilities for more than 2 inches that
reach 70-90% in central IA, with locally up to 4" possible.

Farther downstream, as the low coalesces to be more pronounced and
has stronger frontal features accompanying it, a period of strong=20
upslope flow will occur in its wake across the Central=20
Appalachians. The DGZ is shallow and thin, but pronounced lift into
it will result in a period of heavy snow, especially late Friday=20
into Saturday, from the Laurel Highlands through southern WV.=20
During this time, moisture will be enhanced as well from upstream=20
Great Lakes connection, enhancing the potential for heavy=20
accumulations, and WPC probabilities D2 are high (>70%) and=20
moderate (30-50%) D3 for 4+ inches of snow, leading to local maxima
around 12 inches by the end of the forecast period.

Finally, although accumulations are expected to be minimal, at
worst, some light snow may crest the Appalachians and move across
the Mid-Atlantic states Friday afternoon bringing at least
conversational snow to the areas around Washington, D.C.


...Northern High Plains...
Day 3...

A stripe of heavy snowfall is becoming more likely Saturday from
central MT through western SD. This snowfall will be associated
with a slowly advancing warm front topped by increasing divergence
downstream of a potent shortwave digging into the Four Corners
region. While jet dynamics are weak during this time, the
downstream moist advection on impressive 280-290K isentropic ascent
should expand precipitation in a NW to SE fashion late Friday night
and through Saturday. Robust 850-700mb fgen into this moistening
column will help intensify ascent resulting in heavier snow rates,
but at this time the intensity of this snow is still uncertain due
to modest DGZ depth probabilities and a lack of ideal overlap
between the best ascent and the DGZ. Still, a band of moderate to
heavy snow is becoming more likely, and WPC probabilities have
increased to 30-50% for 4+ inches across north-central MT.
Additional increases are possible with later model runs.


...Central Plains through the Mid-Atlantic...
A significant winter storm is becoming more likely Saturday through
Monday, with the onset of light freezing or frozen precipitation
beginning Saturday evening across Kansas. At this, time, WPC
probabilities for significant snow or ice are minimal through the
end of D3, but heavy snow and significant icing is expected into D4
and beyond. For this system, WPC has initiated Key Messages which
are linked below, and more information can be found in the WPC
extended range discussion as well.


Weiss



...Winter Storm Key Messages are in effect. Please see current=20
Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!6t_Z-BmJBuOGYk-H-BCTPT4b9gUF9fw1sJpzXIfwuaImT= 7jkQ-k9RwmMDpxmoZeDuE_QGuno2GwEVAA-a0RA2L4RUGs$=20

$$

=3D =3D =3D
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FOUS11 KWBC 020840
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
340 AM EST Thu Jan 2 2025

Valid 12Z Thu Jan 02 2025 - 12Z Sun Jan 05 2025

...The West...=20
Days 1-3...

Persistent troughing across the Pacific will maintain broad SW flow
into a modest ridge along the coast until Saturday when a
pronounced closed low digs eastward towards OR forcing a trough to
move inland to the Great Basin.=20

Periodic impulses shedding through the flow will move into a region
of greater mid-level divergence D1, leading to broad ascent and=20
increasing precipitation coverage. For today, this will be most=20
pronounced along a low-level warm front/baroclinic gradient lifting east/northeast from OR into the Central and Northern Rockies. This
will result in a swath of precipitation, primarily as snow, but=20
with snow levels rising to as high as 6000 ft in southern=20
OR/northern Great Basin, with a sharp gradient to less than 1000 ft
into the Northern Rockies and Columbia Basin.

Heavier snowfall is likely at the very end of D1 into much of D2=20
as more substantial moisture and ascent spread onshore Friday into=20
Saturday. This more enhanced forcing/moisture is associated with=20
the closed low moving onshore and causing the entire trough axis to
pivot eastward. Impressive lift will be aided by the LFQ of a=20
modest, but well placed, upper jet streak, and there is likely to=20
be strong deep layer lift expanding across much of the Pacific and=20
Interior Northwest Friday night, shifting as far as the Northern=20
High Plains by Saturday night. The most impressive ascent will=20
again be collocated with a secondary warm front lifting northward=20
from OR to Canada on Friday, leading to enhanced WAA/isentropic=20
ascent within the broad synoptic lift regime. While overall omega=20
is modest, a swath of moderate to heavy snow is likely from the=20
mountainous areas of northern CA through OR, WA, and into the=20
Northern Rockies, with snow levels generally rising to 4000-6000=20
ft. This is reflected by WPC probabilities which suggest a high=20
risk (70-90%) for more than 6 inches of snow in the Cascades and=20
Northern Rockies, with more general onshore flow leading to upslope
snow across the typical terrain features D3 from the Sierra,=20
Cascades, and into the Northern Rockies again.

Additionally, the warm advection overrunning cold high pressure
centered over Saskatchewan is progged to leave low-level cold
easterly flow from the Columbia basin into the Columbia Gorge.
Although eventually this will be scoured out, the setup appears
favorable for a prolonged period of moderate freezing rain in the
eastern foothills of the Cascades and into the Columbia
Gorge/Basin. The heaviest icing is expected along the Hood River
Valley and into the Gorge which will take a longer time to scour=20
out the cold air, and this is reflected by WPC probabilities that
reach as high as 30% for 0.1" of ice accretion.


...Great Lakes and Northeast...=20
Days 1-3...

Surface low pressure moving across eastern Maine early this morning
will usher in a long duration lake effect snow (LES) event into=20
the weekend. This surface low will be pushed northeast in response=20
to a potent vorticity maxima rotating through the negatively tilted
trough, which eventually swings northwestward towards the Hudson
Bay as a larger upper level low and stalls. This allows for strong
and persistent cold northwesterly flow over the currently ice-free
Great Lakes and substantial snowfall likely associated with the=20
LES, especially D1 and through D2 as lake-induced instability=20
climbs above 500 J/kg. The pattern will support heavy LES in the=20
favored W/NW snow belts from the U.P., across the L.P., and then=20
downstream to east of Lakes Erie and Ontario, with a favorable=20
upstream moisture connection from Lakes Superior and Huron=20
enhancing LES off of Lake Ontario. LES is expected to be most=20
widespread and heavy through Friday, when 1-2"/hr snows, possibly=20
greater at times, cause WPC probabilities to be high (>70%) for 6+=20
inches downstream of most Lakes, and reach above 90% for 8+ inches=20
east of Lake Ontario D2. Total snowfall of multiple feet is likely,
with the highest along the Tug Hill Plateau. LES will continue=20
through D3 with just slightly lesser intensity and coverage as=20
reflected by WPC probabilities for an additional 6+ inches being=20
above 70% in just narrow channels near the Chautauqua Ridge and the
Tug Hill Plateau.


...Midwest through Central Appalachians and Mid-Atlantic...=20
Days 1-3...

An inverted surface trough sandwiched between a retreating high
pressure over the Southeast and an incoming strong Canadian high
pressure from Alberta will become a focus for a developing wave of
low pressure and an associated stripe of snowfall from west to
east. The sharpening of this trough will initially be in response
to a subtle vorticity lobe swinging through the broad trough across
the east, ejecting from the High Plains into the Upper Midwest
today. Immediately following this impulse, a potent jet streak=20
will drop southeast into the Northern/Central Plains while=20
amplifying to 150 kts, providing favorable overlap for deep layer=20
ascent. This will stem cyclogenesis along the inverted trough, with
increasing downstream warm/moist advection surging moisture along=20
the 280-285K isentropic surfaces into the Upper Midwest. This WAA=20
will help deepen the DGZ to the east, while 925-850mb fgen=20
increases, although only modestly.=20

This will result in a stripe of moderate snow from Iowa this
morning through the Ohio Valley, where at least brief snowfall=20
rates to 1"/hr are possible, leading to a general 1-3" of snowfall accumulation, with locally up to 4" possible.

Farther downstream, as the low coalesces to be more pronounced and
has stronger frontal features accompanying it, a period of strong=20
upslope flow will occur in its wake across the Central=20
Appalachians. The DGZ is shallow and thin, but pronounced lift into
it will result in a period of heavy snow, especially late Friday=20
into Saturday, from the Laurel Highlands through southern WV.=20
During this time, moisture will be enhanced as well from upstream=20
Great Lakes connection, enhancing the potential for heavy=20
accumulations, and WPC probabilities D2 are high (>70%) for 6+=20
inches of snow, leading to local maxima around 12 inches by the end
of the forecast period.

Finally, although total accumulations are expected to be minimal, some
snow showers and snow squalls may crest the Appalachians and move=20
across the Mid- Atlantic states Friday afternoon as the vorticity=20
max dives southeastward. The late afternoon timing of this vort max allows
for steep lapse rates from the sfc all the way up to 500 mb and the
potential for snow squalls to develop and potentially contain
1-2"/hr snowfall rates. Surface temperatures in the Washington=20
D.C. to Baltimore region will start out above freezing, but areas=20
just north and west may be right around the freezing mark. Either=20
way 850 mb temps of -5 to -6C will allow for the heavier precip=20
rates to quickly fall as snow and surface temperatures to quickly=20
crash as well. This may lead to potentially hazardous travel and=20
rapidly deteriorating road conditions for the Friday evening=20
commute depending on the strength of the snow showers/squalls and=20
where exactly they impact in the Mid-Atlantic.


...Northern High Plains...
Day 3...

A stripe of heavy snowfall is becoming more likely Saturday from
central MT through western SD. This snowfall will be associated
with a slowly advancing warm front topped by increasing divergence
downstream of a potent shortwave digging into the Four Corners
region. While jet dynamics are weak during this time, the
downstream moist advection on impressive 280-290K isentropic ascent
should expand precipitation in a NW to SE fashion late Friday night
and through Saturday. Robust 850-700mb fgen into this moistening
column will help intensify ascent resulting in heavier snow rates,
but at this time the intensity of this snow is still uncertain due
to modest DGZ depth probabilities and a lack of ideal overlap
between the best ascent and the DGZ. Still, a band of moderate to
heavy snow is becoming more likely. WPC probabilities have=20
increased to 50-80% for 4+ inches across north-central MT, with
40-50% chances for at least 6 inches by Saturday night.


...Central Plains through the Mid-Atlantic...
A significant winter storm is becoming more likely late Saturday=20
through Monday, with the onset of light freezing or frozen=20
precipitation beginning Saturday evening across Kansas. At this,=20
time, WPC probabilities for significant snow are minimal through=20
the end of D3 (Sun morning), but heavy snow is expected to be
developing at that point and continuing to into D4 and beyond. The
upper low swinging through the Great Basin on Saturday is expected
to eject into the central Plains Saturday night while closing off
and strengthening within a developing favorable duel jet structure.
Strong WAA will develop and blossom the precipitation shield across
the central Plains and Mid-Mississippi Valley Saturday night while
also promoting a warm nose to lift northward. This creates a
favorable setup for mixed ptypes and wide swath of sleet/freezing=20
rain. Guidance has trended slightly north with QPF and the mix zone
in recent runs as is common for WAA regimes with a surface High=20
remaining displaced to the NW. It's possible the warm nose trends=20
farther north as hires CAMs become available, also impacting areas
downstream by D4. WPC ice probabilities have increased by the end=20
of D3 and are 20-40% for at least 0.1" of freezing rain accretion=20
from central KS to southern Missouri. For this system, WPC has=20
initiated Key Messages which are linked below, and more information
can be found in the WPC extended range discussion as well.


Snell/Weiss




...Winter Storm Key Messages are in effect. Please see current=20
Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late= stKeyMessage_1.png__;!!DZ3fjg!9e2ebve2eyI5FZ_k_MClbg0vZkP7WKORJo4yZsMhMzdC3= nSH59X0LZ-9zXmTzUtaiBfbPJTbQpAusH7hnwanYsYEEwQ$=20

$$

=3D =3D =3D
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