Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
336 PM EST Sat Dec 07 2019

Valid 00Z Sun Dec 08 2019 - 00Z Wed Dec 11 2019


...Mountain West...
Days 1-2...

A positively tilted long wave trough will shift southeast across
The West through Monday. Widespread precip with mountain snows are
expected ahead of the wave. Day 1 WPC probabilities for six inches
are high for the Sierra Nevada, central, northern Rockies, and
higher ranges in between. Probabilities for 18 inches on Day 1 are
high for the High Sierra and Tetons. The Day 2 focus is with the
southern stream trough over the Desert Southwest with
probabilities for six inches contained to the highest four corners
states mountains (mainly CO).


...Northern Plains into the Central Great Lakes...
Days 1-3...

A northern stream shortwave trough embedded in an amplifying
longwave trough will shift southeast from the Canadian Rockies
tonight and cross the Northern Great Plains Sunday through Monday
before lifting northeast from the Great Lakes low Monday night.
The coldest air of the season will plunge in behind this wave.
Ahead of the northern stream trough, a wave of low pressure will
develop along a cold front and race quickly eastward spreading a
swath of snowfall from the High Plains of MT eastward across the
Dakotas to northern Michigan. Modest mid-level frontogenesis and
weak left exit jet level diffluence will combine to produce
moderate snow. A northward trend continues with the 12Z guidance.
The rapid progression of this feature should limit total
accumulation, though dendritic snow is expected. WPC probabilities
are generally moderate for four or more inches on Day 1 in
northeastern MT/western ND, over southeastern ND, central MN to
lake Superior on Day 2 (with moderate probabilities for six inches
over northern WI), and over northern MI on Day 3 when lake effect
from the particularly cold air commences.


...Southern High Plains...
Day 3...

The southern stream portion of the trough shifting southeast
through The West tonight through Monday will cross AZ Monday and
allow lee side surface low development near the Big Bend of TX
Monday night. Flow around the low along with height falls aloft
should allow some snow development over the higher terrain of west
Texas and southeast New Mexico into Tuesday. As of now this looks
rather light, but climatologically fairly rare and there is
potential for this low to develop a bit quicker than the current
guidance suite suggests and allow for some banded snow with
moderate snow rates.

For Days 1-3, the probability of significant icing (0.25-inch) is
less than 10 percent.

Jackson