Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
324 AM EST Sun Nov 14 2021

Valid 12Z Sun Nov 14 2021 - 12Z Wed Nov 17 2021


...Pacific Northwest to the Northern Rockies...
Days 1-3...
Another atmospheric river event coincident with an
approaching/lifting surface warm front will continue to bring
heavy precipitation across the orographically-favored regions of
western Washington today. Snow levels at or above 8000 ft for most
regions will remain high until tomorrow when a lead weak shortwave
starts to chip away at the upper ridge, followed by a stronger
shortwave and weakening upper low in the southern Gulf of Alaska
later tomorrow. This will finally bring in a stronger cold front
that will push the moisture plume southeastward as it weakens and
splits, in concert with lowering snow levels down to around 2000
ft by Tuesday morning as precipitation ends over Washington, with
most passes seeing accumulation.

To the east, moisture spreading east of the Cascades along and
atop a surface boundary will mostly be rain with high snow levels
today. By tomorrow into Tuesday, lowering snow levels will bring
some locally modest snows into northwestern Montana (Glacier Park)
as the cold front over Washington continues southeastward. The
front will continue through the region as low pressure deepens
over southern Canada, bringing a burst of snow to northwestern
Wyoming early Tuesday.

Great Lakes and Northeast...
Days 1-2...
A potent clipper system will continue to push through Wisconsin
into Michigan today after dropping up to 10 inches of snow over
northern MN. Generally light snow under a few inches is likely for
most locations days 1-2, with the exception being downwind of the
Great Lakes in favored areas that also have a bit of elevation
gain (e.g., northwestern PA) in addition to parts of the central
Appalachians in eastern WV and western MD where upslope snow may
squeeze out several inches. The central U.P. of Michigan east of
Marquette and parts of the western Adirondacks into the Tug Hill
have the highest probabilities of at least 4 inches of snow
through Tuesday (above 60-80%). The system will translate offshore
Maine by day 3 as another low offshore becomes dominant, ending
any snow over the Northeast.

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

Fracasso