Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
135 PM EST Sun Nov 12 2023

Valid 00Z Mon Nov 13 2023 - 00Z Thu Nov 16 2023

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

A shearing mid-level shortwave trough will race eastward and lift
onshore WA state early Monday morning. This will help drive a fast
moving surface cold front into the Pacific Northwest, with
downstream precipitation likely through the overlap of this
ascent. At the same time, the tail of a Pacific jet streak will
move overhead, although it appears the best upper diffluence
associated with this feature will lag the mid-level PVA and
surface frontal convergence. Additionally, while a stream of
elevated PWs above 0.75 inches lifts northeast ahead of the front
on WAA, this is only marginally anomalous reflected by near-normal
anomalies from the NAEFS ensemble tables. This indicates the more
impactful result of this WAA will be snow levels rising to around
5000 ft ahead of the front, and while they will crash to 3000 ft
behind it, this will also occur with rapid drying bringing an end
to any snowfall by D2. This transient event should result in
modest snowfall accumulations, especially in the northern
Cascades, where WPC probabilities for more than 4 inches are above
50%.

After a break Monday night - Tuesday, a secondary vorticity maxima
will drop southeast out of British Columbia and shift eastward
into confluent flow over the Northern Rockies. The confluent flow
in this region will help enhance moisture as elevated mid-level RH
merges from both the northern and southern streams, resulting in a
ribbon of elevated PWs exceeding 0.5" pivoting into the Northern
Rockies. At the same time, a stationary front will waver in the
vicinity driving additional ascent through overrunning, and it is
possible that a wave of low pressure may form along this boundary
on Wednesday. The snowfall forecast has increased for elevations
above 5000ft around Glacier NP, and WPC probabilities are 10-20%
for more than 4 inches across the Selkirk Mountains and near
Glacier NP on D3.

Weiss