Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
333 PM EST Sun Dec 23 2018

Valid 00Z Mon Dec 24 2018 - 00Z Thu Dec 27 2018


...Western U.S....

Mid-level energy embedded within progressive onshore flow will
promote periods of snow across the Olympics, Cascades and portions
of the northern Rockies, producing additional heavy accumulations
tonight into Mon.

The focus for heavy snow will begin to shift further south on Mon,
as a well-defined shortwave dives southeastward into northern
California Mon night. This is expected to bring locally heavy
accumulations from the southern Cascades into the northern Sierra,
with significant accumulations also possible further to the east
into the higher elevations of the Great Basin. As the shortwave
continues to dig southeast, with an upper upper low closing off
over Southern California and the Southwest, snows will shift
further south and east, reaching into eastern Arizona and western
New Mexico and western Colorado late Tue into Wed.


...Central/Northern High Plains...

As shortwave energy crosses the Four Corners region Tue night/Wed,
cyclogenesis will commence across the southern High Plains, with
an intensifying area of low/mid-level frontogenetic forcing taking
shape north of the surface low. Additionally, northern stream
shortwave energy crossing the northern Rockies on Wed may result
in some degree of phasing interaction with the southern stream
wave (the degree to which this occurs remains a bit uncertain due
to variable model timing on the northern stream energy). Model
differences with the phasing play a role in the speed with which
the southern plains cyclone takes shape and the intensity of warm
air advection ahead of the system, which will play a significant
role in p-type across portions of the central/northern plains and
Midwest. The event across the High Plains will be ramping up
through/beyond the end of the forecast period (00Z Thu), with the
potential for a band of moderate to heavy snows taking shape
across portions of NE and SD by Wed afternoon. Day 3 probabilities
show moderate chances for exceeding 4 inches and slight chances
for exceeding 8 inches across portions of these areas.


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


Ryan/Pereira