179
FOUS11 KWBC 230738
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
238 AM EST Sun Feb 23 2025

Valid 12Z Sun Feb 23 2025 - 12Z Wed Feb 26 2025

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

Long duration atmospheric river (AR) begins before the forecast
period, and continues through Tuesday. This AR is driven by
confluent mid-level flow overlapped by multiple impressive Pacific
jet streaks, resulting in pronounced onshore flow of moisture. The
accompanying IVT has a high probability (>80% from ECENS and GEFS)
of exceeding 500 kg/m/s through Monday, with a slow wane
thereafter. Within this plume of moisture, forcing for ascent will
be periodically enhanced by shortwave troughs rotating through the
flow as impulses shed from a strong closed mid-level low out over
the Pacific Ocean. Each of these impulses will produce enhanced
ascent to expand and intensify the accompanying precipitation,
leading to rounds of heavy snow from the Cascades through the
Northern Rockies. However, the WAA within the AR will lift snow
levels to as high as 8000 ft, before latitudinal variations develop
on either side of a stalled front.

For D1, a closed 500mb low pushing onshore British Columbia will
pulse a lobe of vorticity into WA state, accompanied favorably by
coupled jet streaks. This will combine with a cold front pushing
eastward to enhance ascent and drive rounds of heavy precipitation
from northern CA through the Northern Rockies, with upslope flow
into terrain features resulting in the heaviest accumulations of
precipitation. Snow levels during this time will rise to generally
6000-8000 ft, keeping snow confined above pass levels. However,
where it does snow, accumulations will be significant as reflected
by WPC probabilities that indicate a high risk (>70%) for 8+ inches
in the Sawtooth, Northern Rockies near Glacier NP, and the Tetons
of WY.

This first front will then become elongated west-east as the
mid-level pattern becomes flattened in response to a sharper trough
approaching from the West. This will allow some colder air to drain
southward but remain confined generally to WA, northern ID, and MT,
with a sharp gradient in snow levels existing along this boundary
(3000 ft north, 6000 ft south). This will be relatively short lived
however, as the stronger trough axis drives a potent closed low and
accompanying surface low towards the WA/OR coast late Monday. This
will once again result in enhanced ascent through WAA, PVA, and
upper diffluence, with a second surge in precipitation expanding
east across the area. Snow levels climb again to as high as 6000 ft
in many areas late Monday into early Tuesday, before the associated
cold front drives eastward by the end of the period. While this
will cause snow levels to fall quickly, it will also result in
rapid drying of the column, bringing an end to the heavy
precipitation.

D2 snowfall is likely to be heaviest along the spine of the
Cascades and into the Olympics of WA, where WPC probabilities are
high (>90%) for 8 inches, and locally 1-3 feet is possible in the
highest elevations. During D3 the heaviest precipitation spreads
inland as far as the Northern Rockies and into the vicinity of
Yellowstone NP, but the heaviest accumulations are likely again
across the WA Cascades where WPC probabilities are above 50% for an
additional 8+ inches of snow.


Weiss


$$