Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
311 AM EST Thu Feb 20 2020

Valid 12Z Thu Feb 20 2020 - 12Z Sun Feb 23 2020


...Southern Appalachians and Mid Atlantic...
Day 1...
The models show widespread precip across the southern Appalachians
to the southern mid Atlantic and the Carolinas.
The mixed precip event starts as rain in most lower elevation
stations and as the low pressure develops in the coastal waters,
cold air sinks south across these regions.  A mid level
frontogenesis maximum crossing the southern Appalachians and the
Carolinas focuses precip across these areas.  One maxima of
several inches of snow is expected in the mountains near the NC/TN
border.  A second maximum is expected in northeast NC, where
continuing mid level frontogenesis this evening once it has become
cold enough for snow supports a several hour period of snow. This
should result in several inches of snow in northeast NC to the VA
border inland from the coast.

Near the coast from Cape Hatteras down to Wilmington, the shorter
duration the precip will fall as snow, limiting accumulations. The
event ends close to the beginning of day 2 as the low pulls
offshore.
WPC probabilities for 4 inches are around 50% in the mountains,
and in northeast NC to the southeast VA border.

...Great Lakes...
Day 1...

Northwest flow with lee shore moisture convergence supports snow
showers crossing Lake Ontario into western New York early today. 
The strength of the low level convergence weakens this afternoon
as the low level flow becomes anticyclonic.
The dry air aloft advecting across the region tonight should
continue to decrease snow shower coverage.
The best chance for a localized band of briefly have snow snow
would be this morning.

...Southern Great Basin...
Day 3...

An upper low and associated surface wave will approach the
Southern California coast Friday before moving onshore Saturday
and into the southwest. Moisture will spread inland on southerly
warm/moist advection. Moisture becomes significant as evidenced by
PWAT anomalies reaching +2 to +3 standard deviations above the
climo mean.
Upper level divergence in the left jet exit region crosses along
the track of the 700 mb low across the southern to central Ca
Sierra Nevada, ranges of central NV and into the ranges of UT. 
Upslope flow in the aspects facing the low0mid level southwest
flow favors an extended period of snow in the San Juans of
southwest CO.  The high snow levels confine higher WPC
probabilities for 6 inches to the higher elevations of the CA
Sierra Nevada, Nevada ranges, southern Wasatch, and San Juan
mountains of CO.

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

Petersen