Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
330 AM EST Mon Dec 07 2020

Valid 12Z Mon Dec 07 2020 - 12Z Thu Dec 10 2020

...Pacific Northwest...
Days 2/3...

A zonally oriented Pacific jet streak will push quickly across
southern Canada Monday into Tuesday, leaving its RRQ across the
PacNW during the latter half of D2 and into D3. Immediately in its
wake, a shortwave will race eastward to move onshore WA state
Wednesday, and the ascent provided through the two will produce
precipitation across the region. At the same time, a weak wave of
low pressure beneath the shortwave will advect east, trailing a
cold front into the NW as well. This will cause snow levels to
gradually fall so moderate snow will spread from the WA Cascades
Tuesday into the Northern Rockies Wednesday. WPC probabilities for
4 inches for either day is less than 30%.

Additionally, cold high pressure settled in the lee of the
Cascades will only slowly retreat as the surface low moves
eastward. This will trap some low-level cold air in the Columbia
Basin of eastern and central WA. As precip spreads across the
region from the west, light freezing rain is possible, and WPC
probabilities now indicate a 40-50% chance for 0.1" of accretion
on D2.


Weiss