087
FOUS11 KWBC 210623
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
123 AM EST Fri Feb 21 2025

Valid 12Z Fri Feb 21 2025 - 12Z Mon Feb 24 2025


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

An increasingly active pattern across the Pacific and into the
Northwest CONUS will drive two rounds of wintry precipitation
across the region through the weekend.

The first of these will approach this evening as a modest shortwave
impulse sheds from a larger closed low over the Pacific and moves
onshore by 00Z/Saturday. This will push a weakening cold front
onshore from an occluded low filling across British Columbia.
Modestly pinched mid-level flow will help surge moisture into the
region ahead of this front, characterized by PWs above the 90th
climatological percentile across WA according to NAEFS. The
accompanying ascent through WAA and then convergence along the
front itself will help expand precipitation across the region, but
with snow levels rising to 5000 ft west of the Cascades, and rising much
more slowly to the east. This will keep snow accumulations
generally above pass level (Washington Pass the exception), with
light icing possible across eastern WA state/the Columbia Basin.
Duration and intensity of precipitation D1 into D2 is likely to be
limited, so WPC probabilities for more than 4 inches of snow above
50% are confined to just the highest peaks of the WA Cascades.

After a brief break in precipitation Saturday, a more substantial
plume of moisture pivots onshore beginning Saturday evening/night,
driven by an atmospheric river (AR) which has high probabilities
(>90%) in both the ECENS and GEFS for IVT to exceed 500 kg/m/s.
This plume will be advected eastward thanks to increasingly
confluent flow just west of the coast and downstream of a
sharpening closed low, aided by a strengthening upper level jet
streak. By 00Z Sunday, IVT according to NAEFS will exceed the 99.5
percentile within the CFSR database, and this will continue to
spread across WA, northern ID, and into MT/WY by the end of the
forecast period. This impressive moisture combined with strong
synoptic ascent will result in an expansion of heavy precipitation,
but with snow levels concurrently rising to 6000-8000 ft. This
indicates that despite precipitation being very heavy at times,
most of the snow will be confined to the highest elevations and
well above pass levels. This is reflected by WPC probabilities D2
that are 70% or more for 6+ inches in the higher WA Cascades, which
then expand eastward, reaching 70% or more for 6+ inches across
portions of the Sawtooth, Northern Rockies, and Grand Tetons D3.


Weiss


...Extreme Cold Key Messages are in effect. Please see current Key
Messages below...

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_2.png

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