Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
409 AM EST Thu Jan 14 2021

Valid 12Z Thu Jan 14 2021 - 12Z Sun Jan 17 2021

...Northern Plains and Upper Midwest...
Days 1-2...
An upper level trough positioned over the northern Plains this
morning will continue to amplify as a potent shortwave digs
southeast across the central Plains into the mid Mississippi
valley on Thursday, with a closed low developing to the north
across the upper Mississippi by late in day.  Warm advection
precipitation currently moving east from the Dakotas is expected
to fall as a wintry mix across portions of the upper Mississippi
valley this morning, before changing over to all snow as cold air
aloft arrives.  A weakening occlusion to the north of a
redeveloping surface low centered over southeastern Minnesota and
southwestern Wisconsin is expected to help focus light snows
across northern Minnesota into late Thrursday.  As the upper low
continues to deepen, heavier snows are expected to develop within
the associated deformation band setting up across southern
Minnesota, Wisconsin and Iowa by late Thursday into the overnight.
 The 00Z models made a notable shift south and southwest, with
probabilities for significant snow accumulations shifting farther
south across the region.  For the Day 1 period (ending 12Z
Friday), the latest WPC PWPF shows high probabilties for snow
accumulations of 4-inches or more covering a large portion of
southern Minnesota into western Wisconsin and and northern Iowa. 
The system is expected to gradually weaken and move east on
Friday.  However, some portions of the area, particularly southern
Minnesota and Iowa, could see at least a few more inches before
the system departs.

...Central Appalachians and Northeast...
Days 2-3...

The closed low from the Midwest will broaden and begin to lift
east/northeast Friday into Saturday, spreading waves of
precipitation eastward both along the front and associated with
vorticity impulses rotating around the center. The primary surface
low is expected to occlude over Michigan, with a secondary wave
developing farther to the east along the triple point, centered
near New Jersey/Long Island by early Saturday.  Steepening lapse
rates beneath the mid-level vortex will combine with waves of
energy to produce periods of snow showers over the central
Appalachians, with increasing northwest flow driving upslope flow
into the favored windward slopes of the central to southern
Appalachians.   This is likely to produce accumulations of at
least an inch or two across portions of the favored terrain. 
Further east, warm air advection precipitation spreading ahead of
the secondary low will move across upstate New York and New
England.  General model consensus shows the low tracking near the
Vermont-New York border -- supporting mostly snow across the the
Adirondacks, while most areas across northern New England can
expect some transition to rain on Saturday.

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

Pereira