Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
413 AM EDT Thu Oct 21 2021

Valid 12Z Thu Oct 21 2021 - 12Z Sun Oct 24 2021

...Pacific Northwest and Northern California...
Days 1-3

An intense shortwave trough is expected to move onshore northern
California Friday as it rounds the base of a sharp mid-level
trough. Favorable dynamics and ingredients are coming together for
a widespread precipitation event characterized by strong upper
divergence and warm air advection. This will help drive heavy
upslope snowfall, mainly confined to the highest elevations of the
Oregon and Washington Cascades and northern California
Shasta/Siskiyou mountains thanks to high snow levels of 9-10+ kft
initially. Behind the front, snow levels do drop to around 6-7 kft
but the majority of precipitation is expected along/ahead of the
front. As a result, WPC probabilities for 4 inches are above 50%,
mainly from late tonight through Friday morning and also confined
to the highest terrain, with Mt. Shasta possibly receiving nearly
2 ft of snow. Another system passes through northern California
and the Pacific Northwest late Saturday into Sunday //day 3//
bringing a renewed round of moisture and lift to the region. This
will keep unsettled weather in place with accumulations again
confined to the higher elevations. WPC probabilities for 4 inches
or more are as high as 20-30%, generally above 6000 ft from the
Olympics southward along the Cascades and towards Mt. Shasta.  

For Days 1-3, the probability of significant icing is less than 10
percent.

Taylor