Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
401 AM EST Thu Nov 22 2018

Valid 12Z Thu Nov 22 2018 - 12Z Sun Nov 25 2018

...Western U.S. to the northern Plains...
Days 1-3...

The leading shortwave within a broad upper level trough will
continue to move across the western U.S., supporting widespread
light to moderate, with some locally heavy snows, from the
Cascades and Sierra to the northern and central Rockies on
Thursday.  As the leading wave reaches the Rockies, models show a
vigorous upstream shortwave with a subtropical moisture connection
entering the Pacific Northwest Thursday evening.  This wave is
expected to move quickly inland as another shortwave, dropping
south from the Gulf of Alaska brings colder air into the Northwest
on Friday.  With snow levels falling, ample moisture along with
favorable forcing aloft are expected to support heavy snow
accumulations along the Rockies from Idaho to western Wyoming and
northern Utah.  Some of the heaviest accumulations are expected to
center along the northern Wasatch and western Uintas mountains,
with WPC probabilities showing three day totals likely to exceed
two feet across portions of the region.  The shortwave dropping
into the Northwest on Friday is forecast to continue to dig
further southeast into the central Rockies on Saturday, supporting
some locally heavy accumulations across the region, before lighter
snow accumulations begin to spread out across South Dakota and
Nebraska and perhaps as far east as Iowa and southern Minnesota
Saturday evening into the overnight hours. 

...Southern Appalachians to southern Mid-Atlantic...
Day 2...

Models continue to show a wintry mix with some light ice and snow
accumulations along the southern and central Appalachians ahead of
a mid-level trough approaching the region Saturday morning.  A
cold surface high is expected to support subfreezing temperatures
as the onset, however strong southerly low level winds and a
retreating high will likely encourage a rapid changeover to rain. 
WPC probabilities remain near zero for ice accumulations of 0.25
inch or more, but do reach 20-30 percent for 0.1 inches across
western North Carolina.

Otherwise, the probability of significant ice (0.25-inch or more)
is less than 10 percent.

Pereira