AWUS01 KWNH 131726
FFGMPD
CAZ000-140300-
Mesoscale Precipitation Discussion 0035
NWS Weather Prediction Center College Park MD
1225 PM EST Thu Feb 13 2025
Areas affected...Coastal Central and Southern
California...Foothills of Sierra Nevada...
Concerning...Heavy rainfall...Flash flooding likely
Valid 131730Z - 140300Z
SUMMARY...Strong Cyclone and atypically broad Atmospheric River
will have potential for .75"/hr rates and localized totals of 3-5"
in favored orographic ascent. Flash flooding is likely, especially
in lower FFG of Southern California. Significant flash flooding
may be possible in/near fresh burn scars.=20
DISCUSSION...GOES-W WV suite depicts a broad, fairly symmetric
closed low just off the NW coast of California continuing to bowl
ESE likely crossing the coastline much later tonight. This low is
atypically deep with the 90th to 95th percentile, but in
combination with fairly cold/stronger ridging downstream has
resulted in an atypically broad subtropical moisture plume with
1.25-1.5" total PWat values along upstream along/ahead of the
height-falls and associated cold front. GOES-W Vis/IR suite
denotes a stark cloud line with slightly anafrontal to the
southeastward cold front and moisture surge. Within the thick
stratus plume, 850-700mb flow is in the 50-65kt range and is
currently orthogonal to the Santa Lucia and Diablo Ranges,
presenting strong IVT/orographic convergence along the range.
...Central California/Sierra Nevada Foothills...
The IVT values are 4-5 standard anomaly units above seasonal
average and range about 700-900 kg/m/s. Rates have been slowly
increasing, with 3hr totals reaching 1-1.25" but will likely
increase with an additional .5-.75"/hr for maybe 1-2 hours as the
plume shifts southeastward. Totals already have reached 3-4" in
the Santa Cruz and northern Santa Lucia resulting in rapid stream
rises and some localized reports of flash flooding. The
additional 2-3" over the next few hours will likely result in
spots of 4-6" totals and continue the flash flooding risk.=20=20
Additionally, the plume has saturated the sub-cloud environment in
the San Joaquin Valley to allow for rain to reach the ground as
the core of the plume presses onshore. The warm front will press
through to the foothills and help to increased the depth of
orographic ascent resulting in spots of .75-1"/hr rates before
becoming very heavy snow at or above 7000Kft. This will decrease
with the passage of the plume/cold front and the flux will reduce
accordingly resulting in scattered shallow convective cells and
bands may aggravate the area given those steepening lapse rates,
which may have intense but brief cores of heavy rainfall/small
hail/graupel.
...Southern California...
Unlike further north, there is precedent moist airmass in place
across the California Bight/Channel Islands, surface to 700mb
values are above normal and total PWats are in the 1-1.25" range.
Currently, the warm front has progressed through the highest
terrain of the Transverse Range and winds are veering slightly
while increasing. This will allow for short-term increase in
orographic ascent across the Transverse (particularly east) and
Peninsular Ranges prior the main core of the AR/cold front later
this evening. Rates of .25-.33"/hr will increase as winds slowly
uptick from 15-20 to 30kts by 00z below 7500 Kft across the
ranges. Spots of 1-1.5" are probable to pre-soak the windward
facing topography.=20=20
A few hours prior to 00z, the cold front and AR will round Cape
Conception and winds will have solid veered profile with depth
with 40-50kts of 850-700mb flow fairly orthogonal to the range. A
few hi-Res CAMs suggest 100-250 J/kg may be present from slightly
higher theta-E air in place allowing for a subtle but important
uptick in vertical development that should result in .75"/hr rates
with spotty potential of 1"/hr. The growing concern here, is
while the plume is less orthogonal to the terrain the breadth of
the AR will allow for increased duration of exposure to the higher
flux. As such, below freezing levels, spots of 3-4" are probable
across the western Transverse Range by 03z and likely to be
similar toward the eastern and Peninsular Ranges thereafter. It
is not out of the realm of possibility that spots of 5" are
possible and heavy rainfall will extend to the coast with .5"/hr
rates and totals over 1-1.5" resulting likely flash flooding
conditions through the evening even for urban locations.=20=20
This is of particular concern given recent burn scars are going to
be less tolerable to any rates over .50"/hr let alone any
potential crossings of .75-1"/hr. It is too early to be certain
about any particular canyon/scar, but given hourly rates and
overall totals there is a sizable possibility for significant
flash flooding/debris flows in or near these scars and avoidance
of this prone areas is strongly advised. Please keep close
attention to local statements/warnings from WFO Los Angeles, state
and local emergency managers. It is possible a subsequent
targeted MPD may be required to address ongoing rainfall trends.=20
Gallina
...Please see
https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!5Jo-Vnr9C7RceznyxJQj2mzaqf1eMMqjQu8gci3RUa33MtzZyNzLc3-bkHz0P8kSr9kP= RHKQXeh5NFj5hCkzyP5N6vM$ for graphic product...
ATTN...WFO...HNX...LOX...MTR...SGX...STO...
ATTN...RFC...CNRFC...NWC...
LAT...LON 39472072 39042072 38382025 37511965 37171934=20
36811897 36291868 35631855 35491870 35611899=20
36381942 37042017 37252083 36982108 36442068=20
35832027 35381997 35141959 34871927 34721872=20
34681829 34651797 34351744 34271688 33721646=20
33041646 32571633 32541676 32521721 32761767=20
32821887 33141955 33592012 34292057 35002087=20
35632142 35912176 37062251 37762270 38112257=20
38602206 39002160 39242125=20
=3D =3D =3D
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