Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
410 AM EST Fri Dec 20 2019

Valid 12Z Fri Dec 20 2019 - 12Z Mon Dec 23 2019

Days 1-3...

...Pacific Northwest/Northern Rockies/California....

A long fetch of deep southwesterly flow will continue to support
the transport of tropical Pacific moisture into western Washington
-- fueling heavy precipitation, including mountain snow across
portions of the Olympics and northern Cascades.  Snow levels are
expected to increase  early Friday behind a warm front lifting
through the area, before settling lower once again later in the
day.  For the eastern slopes of the Cascades, especially areas
above 4000ft, additional heavy accumulations are likely, with
several additional feet possible across the higher peaks on
Friday.  Locally heavy accumulations are also possible farther
east across the far northern Idaho and northwestern Montana
ranges.

The threat heavy snow is expected to continue into Saturday, with
several more inches possible over the northern Cascades and far
northern Rockies.  However, as winds begin to back more to the
south ahead of an amplifying wave approaching the coast, this will
begin to limit moisture transport into the region and therefore
expect snows to begin to diminish across the northern Cascades and
Rockies by late Saturday.

With the deeper moisture advection focused farther south, expect
locally heavy snows to develop from the Klamath Mountains in
northwestern California to the Sierra on Sunday.

For Days 1-3, the probability for 0.25 inches of freezing rain is
less than 10 percent.

Pereira