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

Valid 00Z Sat Dec 21 2019 - 00Z Tue Dec 24 2019

Days 1-3...

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

A long fetch of deep southwesterly flow will continue to support
the transport of Pacific moisture into western and northern
Washington and northern Idaho through the day Saturday -- fueling
mountain snow across portions of the Olympics, northern Cascades,
and ranges of northeast WA/northern ID.  Locally heavy
accumulations are also possible in these ranges. 

A wave is forecast to develop over the eastern Pacific west of CA
Sat night and move onshore Sunday, bringing a surge of moisture
and lift into the ranges of northwest CA as the 700 mb low tracks
towards the region near 18z Sunday, and possibly to adjacent OR by
00z Mon.  
Several inches are expected in the ranges of southwest OR as the
low moves through.
A secondary max is expected in the CA Sierra Nevada mountains late
Saturday into Sunday as several surges of moisture and lift cross
the region.

On Mon the closed 700 mb low over OR Sun weakens into an open wave
as it moves across eastern OR and WA and then across northern ID
and northwest MT.  With the system weakening, moisture transport
and lift lead to decreasing amounts and coverage of snow showers
in the northern Rockies.

The focus on Mon is further south in the CA Sierra Nevada.  The
axis of moisture and lift drifts south with time as the upper
trough slowly approaches the coast.  Falling heights lead to a 
period of snow in the ranges of southern CA, such as the San
Rafael Mountains and San Gabriel Mountains. Several inches are
expected in favored upslope areas as the frontal moisture and lift
focus in the terrain.

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

Petersen