FOUS11 KWBC 240647
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
147 AM EST Mon Feb 24 2025

Valid 12Z Mon Feb 24 2025 - 12Z Thu Feb 27 2025

...Pacific Northwest through the Northern Rockies...
Days 1-2...

Ongoing atmospheric river (AR) will intensify and continue through
today and into Tuesday before eroding as the driving trough pivots
inland. Until that occurs, persistent advection of moisture on IVT
exceeding 500 kg/m/s is expected into WA/OR and then spilling as
far east as portions of the Northern Rockies. This AR will be
driven by confluent flow downstream of an impressive closed low
(NAEFS 500-700mb height anomalies below -1 sigma) which will be
overlapped by a strengthening upper jet streak. Weak impulses
shedding eastward from this closed low as it moves into British
Columbia will help to periodically enhance ascent, with additional
lift likely in the vicinity of a surface low moving near the Strait
of Juan de Fuca Tuesday morning. Impressively confluent flow will
keep moisture advection nearly perpendicular to the Cascades and
other N-S ranges, leading to strong upslope ascent as well.

The guidance is well aligned overall with the synoptic and
mesoscale features, leading to high confidence in the evolution of
this event. However, the consensus has trended colder Monday in
areas north of a draped front from west-east near the WA/OR border.
This will keep snow levels more suppressed, generally remaining
around 3000-3500 ft in WA, ID, MT D1 before a slow rise occur late
D1 due to the enhanced WAA as a warm front lifts north ahead of
the surface low. Thereafter, snow levels crash again quickly during
D2 as the accompanying cold front pivots east. Despite the lowering
snow levels D2, this will occur in tandem with a drying column,
which indicates most of the heavy snow should accumulate only at
the higher elevations thanks to rapid wane of precipitation
coverage and intensity by the second half of D2.

WPC probabilities for heavy snow accumulating to more than 6 inches
are high (>90%) in the Olympics and along the crest of the Cascades
D1, with locally 2-3 feet possible at the highest elevations.
Additionally, impactful pass-level snow is likely especially at
some of the higher passes like Washington Pass. During D2, heavy
snow continues across the Cascades but also expands eastward
towards the Northern Rockies. WPC probabilities D2 are high (>70%)
for 6+ inches across the higher elevations of these ranges, before
precipitation winds down into D3.


Weiss



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