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

Valid 00Z Mon Feb 24 2025 - 00Z Thu Feb 27 2025

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

Current assessment of the mid-level WV satellite across the Eastern
Pacific denotes a textbook multi-wave structure within a long axis
stretch of progressive shortwave troughs migrating underneath a
prominent trough axis nestled over the Gulf of Alaska. The current
AR across the Pacific Northwest is driven by the confluent mid-
level flow overlapped by multiple impressive Pacific jet streaks
accompanying each shortwave trough, resulting in a pronounced
onshore flow of moisture centered over the OR/WA coasts, extending
inland. The accompanying IVT has a high probability (>80% from
ECENS and GEFS) of exceeding 500 kg/m/s through tomorrow (Monday),
with a slow wane thereafter. Within this plume of moisture,
forcing for ascent will be periodically enhanced by the
aforementioned shortwave troughs pivoting through the flow as these
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 (50-80%) 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 (>70%) for 8 inches, and locally 1-3 feet is possible in the
highest elevations. A secondary maxima will align within the
highest elevations of Northern ID near the MT/ID border as the
moisture advection and progressive cold front propagate inland
during the back half of D2 leading to moderate probs of 40-70% for
potential of 8+ inches. During the front half of the D3 time
frame, the heaviest precipitation will continue to spread 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. Drier air advecting into the Northwestern U.S
during the latter half of Tuesday will spell the end to the
extended AR impacts that have been in place the past several days.



Weiss/Kleebauer