FOUS30 KWBC 172003
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
403 PM EDT Thu Oct 17 2024
Day 1
Valid 16Z Thu Oct 17 2024 - 12Z Fri Oct 18 2024
...16Z Update...
The probability of rainfall exceeding flash flood guidance is less than
5 percent.
Wegman
...Previous Discussion...
Relative progressive nature of precipitation across Northern NM
will alleviate much of the flash flood concerns this afternoon,
however the very low FFG's in place over each existing burn scar
within the Sangre de Cristos will offer an opportunity for
localized flooding within each impacted zone. Current HREF probs
for >1" of precip is around 50-60% within the neighborhood derived
analysis. This is still very lacking within the EAS prob field
leading to a pattern of less widespread heavier precipitation
totals and more relegated to very localized maxima likely over the
tops of the mountain chain. The best depiction is outlined over the
San Juan Mountains where a majority of the precipitation will fall
as snow or a rain/snow mix mitigating the threat for flash flooding
during the period. This has allowed for a continuation of the nil
ERO across the CONUS, but still a non-zero presence within the burn
scars of Northern NM.
Kleebauer
Day 2
Valid 12Z Fri Oct 18 2024 - 12Z Sat Oct 19 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE FOUR CORNERS REGION THROUGH NORTHERN NEW MEXICO...
...2030Z Update...
No changes were made to the Marginal Risk area in the forecast.
With the synoptic setup described below largely unchanged, the only
detail to add and one of the larger points of uncertainty will be
snow levels...as lower snow levels would mean a greater amount of
the total precipitation falls as snow and thus does not contribute
to flooding. Overall this will be the case further north for much
of UT and CO. Meanwhile for southern AZ and NM lesser amounts of
total rainfall are expected...thus the narrow area of northern NM
and the 4 Corners remains the corridor where there is both adequate
forcing and temperatures are warm enough that when added to the
preponderance of flash flood prone areas such as slot canyons, burn
scars, and any more developed and low-lying areas...isolated flash
flooding remains possible.
Wegman
...Previous Discussion...
Longwave pattern will continue evolving into a fairly prolific
upper trough/closed-low over the Great Basin, slowly migrating
eastward with sights on the Four Corners. Large scale ascent will
become maximized downstream of the mean trough with a pronounced
diffluent pattern in-of the Southern Rockies, including the Sangre
de Cristo and San Juan mountain ranges where heavier precip is
forecast. A cold front will slowly propagate eastward from the
primary disturbance with a return flow setup pressing westward
across the Eastern NM plains on the western flank of a sprawling
surface high east of the Mississippi. A secondary cold front to the
north will plunge southward thanks to a strong ridge of high
pressure developing upstream in wake of the primary western low,
along with a lee side trough developing over the Front Range. The
tandem of boundaries and backed flow will converge over Northern NM
with a strong isentropic ascent pattern initiating later Friday
evening through Saturday AM and beyond.
There has been very little deviation in the guidance to amass much
of a change compared to the previous forecast, so a general
continuity was maintained from the inherited MRGL risk across the
Four Corners into Northern NM. Ensemble guidance is in relative
agreement on the placement of two maxima during the forecast
period; one located Northeast AZ into Southeast UT with the initial
diffluent signature ahead of the evolving upper low over the Great
Basin, and the secondary maxima across Northeastern NM where the
convergence pattern aided by increasing large scale ascent will
generate a period of heavier precip Friday evening into Saturday
morning and beyond. Each areal maxima is similar with regards to
precip magnitude with a general neighborhood 1-2" max with a small
chance at upwards of 2.5" across the western maxima within the best
upper ascent within the nose of the 100kt 250mb jet streak. The
secondary maxima is generally within the last 12-hr window
encompassing 00-12z Saturday with the heaviest QPF footprint
actually highest in D3 (More on that in the D3 period below). In
any case, the very dry antecedent conditions, slot canyons, and
flashier river basins within the Four Corners area will offer a
greater risk of flash flooding with a secondary threat over the
remnant burn scars in Northern NM. This allowed for a maintenance
of the previous MRGL risk.
Kleebauer
Day 3
Valid 12Z Sat Oct 19 2024 - 12Z Sun Oct 20 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS
OF THE SOUTHERN HIGH PLAINS INTO SOUTHEASTERN COLORADO...
...2030Z Update...
...ERO Changes...
The Slight Risk area was nominally expanded east to include the
westernmost counties in the Texas Panhandle, the far western
Oklahoma Panhandle and a few miles into extreme southwestern Kansas
with this update. The surrounding Marginal was expanded west to
include the mountains north of Santa Fe, NM and much of the Texas=20
and Oklahoma Panhandles, as well as far western Kansas.
A new Marginal Risk area was introduced for the Olympic Peninsula
as well as the far northern Cascades of northwestern Washington
State.
...Southern Plains...
Forecast rainfall continues to slowly increase across the southern
Plains this afternoon. A nearly stationary upper low will begin to
tap into increasing Gulf moisture associated with a well
established LLJ across far western TX/OK/ and north up much of the
Plains overnight tonight. By Day 2/Friday the moisture plume will
be well established. The upper level low ever-so-slowly moving east
will gradually tap into more and more of that moisture. Given the
steering flow changes will be slow, the excellent forcing east of
the upper level low and increasing instability will all favor the
development of strong and few severe thunderstorms within the
moisture and instability axis.
These storms will be capable of very heavy rainfall over this
mostly parched/arid area. Due to the LLJ the storms will be pretty
fast-moving towards the NNE. However, the slow moving upper low and
constant resupply of Gulf moisture will strongly favor training
cells across the Slight Risk area, particularly NM. Thus, it's
likely that training storms will develop and move over the same
hard-hit areas causing widely scattered flash flooding.=20
Finally, the 12Z HRRR suggests a line of storms will follow these
individual training cells overnight Friday night. The more
widespread nature of the heavy rain associated with the line is
what's likely to push the more flood prone areas to begin to flood.
The latest guidance suggests the area near the NM/OK border at
greatest risk for the most rain, and therefore the highest threat
for flooding.
...Northwestern Washington State...
A cold front/atmospheric river impacting Vancouver Island and
portions of British Columbia just north of Washington on Friday
will push south into northwestern Washington on Saturday. 1-3
inches of rain are possible in the Olympics and in the Cascades
east of Bellingham Friday, so that by the Day 3/Saturday period,
the full force of the A.R. will move into WA, resulting in local
rainfall totals of 3-5 inches for the day. Integrated Vapor
Transport values at the peak could exceed 900 kg/m/s, which is a
bit stronger than the typical A.R., but nothing major. When added=20
to Friday's rain totals, this will likely be enough rain to result=20
in isolated flash flooding. Even if the highest elevations see=20
snow, the large majority of the rainfall into the mountains will be
in the form of rain. The usual rain shadowing from westerly or WSW
flow should keep much of Seattle and its surrounding suburbs with=20 significantly less (generally under an inch) of rain, so it was=20
excluded from the Marginal Risk area. Further evaluation of the=20
urban areas may be needed with future forecast updates.
Wegman
...Previous Discussion...
Closed upper circulation will slowly migrate eastward with a
persistent diffluent channel focused in-of the Eastern NM High
Plains up into the Colorado Front Range for Saturday. A uni-
directional flow located within the lowest 600mb will persistent
through much, if not all the forecast period allowing for moist
unstable air to be pulled poleward into the confines of the Caprock
and points north. ECENS PWAT anomalies are forecast to run upwards
of 3-3.5 deviations above normal with an M-Climate Percentile
(Forecasts relative to the last 20 years during time of year) that
is peeking into the seasonal max, a testament to the anomalous
nature of the moisture advection regime. With the poleward
advancement of the airmass, regional theta-E's spike with a theta-E
ridge positioning located right over the NM/TX border running clear
into Front Range of CO and nearby KS. This signal is consistent
with a persistent band of rainfall likely to be situated within the
theta-E ridge mentioned above and just out ahead of the slow-moving quasi-stationary front trailing the very slow moving to perhaps stationary
low pressure across Southern CO.
Precip totals of 2-4" with locally higher amounts are increasingly
likely within that corridor between Clovis up through Southeastern
CO with the max likely driven by repeating convection as small
mid-level perturbations rotate around the general upper circulation
and move overhead of the areas above. The only saving grace for a
more significant flash flood potential is the antecedent conditions
leading in are very dry with the 0-10cm relative soil moisture
percentiles running between 5-20% across the area anticipated to
see the most rainfall. Initial rains will be beneficial, but will
add through the course of the period with waves of heavier rain
likely exacerbating areas by later in the period into early Sunday.
The previous SLGT risk was generally maintained with a MRGL
surrounding. Both risks were expanded somewhat on the southern and
northern peripheries to account for a trend in an expansion of
precip latitudinally in either direction. The heaviest will likely
encompass Eastern NM into Southeast CO with these areas likely
trending towards the higher end of SLGT risk if the signal remains
and/or amplifies further.
Kleebauer
Day 1 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_xY3mlGFZzuyBeHSqce0Vr_WIZAL1hOfrnpygIIpLMhr= o6vjki4z8JyU5SbQDGH5bn_SyQj2k7WUW4nmnEH7a3SVDJQ$=20
Day 2 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_xY3mlGFZzuyBeHSqce0Vr_WIZAL1hOfrnpygIIpLMhr= o6vjki4z8JyU5SbQDGH5bn_SyQj2k7WUW4nmnEH7IYH6vXQ$=20
Day 3 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_xY3mlGFZzuyBeHSqce0Vr_WIZAL1hOfrnpygIIpLMhr= o6vjki4z8JyU5SbQDGH5bn_SyQj2k7WUW4nmnEH7qcC7HGo$=20
=3D =3D =3D
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