Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
425 PM EDT Wed Nov 01 2023

Valid 00Z Thu Nov 02 2023 - 00Z Sun Nov 05 2023

...Northern Rockies...
Day 2...

Modest and fast moving shortwave will advect onshore the Pacific
Northwest Thursday and then amplify subtly, but maintain a
positive tilt, as it shifts into the Northern Rockies during
Friday. Downstream ascent through weak height falls and some PVA
will combine with some upper diffluence as weakly coupled jet
streaks develop overhead. This will drive sufficient ascent to
wring out the increasing moisture noted by IVT approaching +2
sigma according to NAEFS on the edge of the weakening AR. The
Pacific source region of this airmass will keep snow levels
elevated, generally 7500-8500 ft, so heavy snow will be confined
to the terrain near Yellowstone and Grand Teton National Parks.
WPC probabilities for more than 4 inches of snow peak at 40-50% on
D2, with 2-day snowfall possibly reaching 12 inches in the highest
terrain.


...Interior Pacific Northwest...
Day 1...

Moisture streaming onshore associated with an atmospheric river
(AR) and associated WAA will spread moisture eastward across WA
state and into ID the latter half of D1. This WAA will overrun
some trapped cold air as Canadian high pressure wedged into the
Cascades only slowly retreats to the east thanks to some leading
mid-level confluence before divergence downstream of modest height
falls moves overhead. Forecast soundings suggest some of the
elevated regions within this wedged high pressure could maintain
surface temperatures cold enough that precip changes from snow to
a period of freezing rain before becoming all rain, resulting in
some light accretions of ice. WPC probabilities for 0.1" of
freezing rain or more are as high as 10-20% in the western
Okanogan Highlands eastward towards the Selkirk Mountains.


Weiss