Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
329 AM EST Mon Nov 15 2021

Valid 12Z Mon Nov 15 2021 - 12Z Thu Nov 18 2021


...Pacific Northwest to the Northern Rockies...
Days 1-2...
The ongoing atmospheric river event will finally start to ease
today as a shortwave and its associated cold front come ashore
Washington. High snow levels this morning with the heaviest
precipitation will drop precipitously as the cold front ushers in
much cooler air, lowering snow levels to below 2000 ft over the
Cascades by tonight as precipitation winds down. WPC probabilities
of at least 6 inches of snow at some pass levels near 3000-4000ft
are around 50-80%. Farther east, the front will bring some snow
and gusty winds to portions of the northern Rockies Monday night
into Tuesday, favoring areas near the Montana/Idaho border
including the Bitterroots with additional snowfall during Tuesday
as a trailing shortwave zips through the region.

...Great Lakes and Northeast...
Day 1...
Broad ascent associated with a clipper system exiting through the
St. Lawrence Valley today will  support decreasing synoptic-scale
precipitation as a coastal system becomes dominant with its
moisture axis into Nova Scotia. A rain/snow mix is forecast for
the lower elevations, with mostly snow across the higher
elevations of the central to the northern Appalachians including
NW PA as well as the Adirondacks. Northwesterly winds behind the
system will support the continuation of lake effect snow showers
across the central/eastern U.P. and northern Lower Michigan, where
WPC probabilities are indicating the potential for localized
additional heavy amounts. Lake effect activity is expected to
return east of Erie and Ontario as winds become northwesterly to
westerly today into tonight. These showers are likely to continue
into early Tuesday before a shortwave ridge shifts east across the
Great Lakes. Both the clipper and the following lake effect may
contribute to several inches of snow across the Tug Hill and
portions of the Adirondacks.

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

Fracasso/Pereira