Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
339 PM EST Tue Dec 24 2019

Valid 00Z Wed Dec 25 2019 - 00Z Sat Dec 28 2019

...Western U.S....
Days 1-3...

The leading shortwave of a broad trough over and off the West
Coast is centered over northwest Mexico this afternoon. This
feature will weaken as it shifts northeast across the Four Corners
states tonight. Ample Pacific moisture of tropical origin will
continue to stream ahead of the trough axis, continuing the
moderate to heavy precipitation currently spreading across the
Desert Southwest. Snow levels will generally be around 6000ft with
this trough. Day 1 WPC probabilities for eight or more inches of
snow (starting at 00Z) are low over the the Mogollon Rim and
Kaibab Plateau and high for the San Juans and Rockies in western
CO. Expect upstream energy moving across the region to generate
additional light mountain snows into Thursday before the next wave
brings another round of precipitation with mountain snows (levels
generally 5000 to 6000 ft) Friday.

A stronger, reinforcing shortwave trough will shift south as a
closed low just off the California coast Wednesday through
Thursday. Generally light snow accumulations are expected across
the northwestern California ranges and the Sierra Nevada tonight
into Christmas, with heavier accumulations beginning to develop
over southern CA later in the day as the upper center continues to
deepen. Strengthening onshore flow, along with increasing
divergence aloft will support moderate to heavy precipitation
spreading southeast across Southern California Wednesday night
through Thursday night. With snow levels generally around 4000ft,
heavy snowfall accumulations are expected. Day 2 WPC probabilities
for 18 inches or more are moderate to high for the highest
mountains of Ventura, Los Angeles, and western San Bernadino and
Riverside counties. Two day snows of 36 to 48 inches are possible
for the highest peaks.

...Icing...
The probability of significant icing (0.25-inch or greater) is
less than 10 percent. However, light icing is possible along a
stationary front in the central Plains tonight, extending across
southern MN Christmas morning (ahead of the leading shortwave
exiting the southwest US).
Also, a warm front lifts into the Northeast Thursday night with
moderate probabilities for a tenth inch of ice in the northern
White Mtns of NH late Thursday night into Friday.

Jackson