Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
250 PM EST Sat Dec 21 2019

Valid 00Z Sun Dec 22 2019 - 00Z Wed Dec 25 2019


Days 1-3...

...Ranges of CA/OR/Great Basin/Northern Rockies...

Tonight and continuing into Sunday the heaviest snows are expected
to focus farther south near an amplifying wave moving inland
across northern California.  Heavy snows are expected to develop
across the Siskiyou, Shastas and Cascade region where the overlap
between ascent and moisture persists tonight into Sunday.  As the
trailing low level front moves inland, snow develops in  the CA
Sierra Nevada on Sunday.  By late Sunday snows are expected to
taper off across much of northern California.  Up to a foot is
possible in the ranges of northwest CA.
On day 2, the 700 mb low weakens into an open wave and deamplifies
as it crosses WA/OR and ID.  Low level warm/moisture advection
ahead of the wave produces light snows in ranges of northeast
WA/northern ID/adjacent northwest MT.

The trailing mid-upper trough is slow to move across CA Monday, so
snows may continue int eh southern CA Sierra and then develop in
the ranges of southern CA like the San Gabriels.
On Monday night, these snows taper as the 700 mb trough starts to
drift east into the southwest.  The moisture plume and lift
crosses the ranges of southern NV/AZ/UT/southwest CO.
Favored upslope areas on southwest flow should receive several
inches of snow.

The probability of significant icing (0.25-inch or greater) is
less than 10 percent.

Petersen