Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
258 PM EST Thu Dec 19 2019

Valid 00Z Fri Dec 20 2019 - 00Z Mon Dec 23 2019

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

Days 1-3...

A large, slow moving deep layer circulation over the northeast
Pacific generates confluent flow with bands of persistent low-mid
level convergence and warm advection combining to produce a threat
for heavy snow Thu night and Fri across the ranges of northern WA,
northern ID, and northwest MT.  The warm advection causes snow
level to rise, so confidence is higher in heavy totals at higher
elevations. A few pockets of light freezing rain or drizzle are
expected near the WA/OR Cascades and ranges of northwest MT where
lift occurs where pockets of drier air seep in aloft.

On day 2, Fri night into Saturday, the continuation of the deep
layer confluent flow and 700 mb convergence and warm advection
causes the event to continue across northern WA, northern ID, and
northwest MT.  As the east Pacific front gets closer to the west
coast, snows start to develop in the OR Cascades and then the CA
Cascades and Siskiyous. 2 day totals of 2-3 feet of snow are
expected in the northern WA Cascades. Heavy snow on the order of 1
to 2 feet also appears likely for the higher terrain of northern
Idaho and northwest Montana.

On Day 3, a wave of low pressure may form on the front as it
approaches CA. The band of confluent low-mid level flow with
convergence and overlapping warm advection on the most of a 50 kt
700 mb jet jet comes into CA, with heavy snow developing at higher
elevations in the ranges of northwest CA, spreading down the CA
Sierra Nevada with time Sat night-Sunday.  The slow moving front
leads to potential for heavy snows to occur in the ranges of
northern CA Sat night-Sun. 
The models are in better than average agreement on the timing of
the upper trough and sfc reflection, with the 12z GFS typically a
few hours faster.

The probability for 0.25 inches of freezing rain is less than 10
percent all 3 days.

Petersen