Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
329 PM EST Tue Nov 16 2021

Valid 00Z Wed Nov 17 2021 - 00Z Sat Nov 20 2021

...Pacific Northwest to the Rockies...
Days 1 and 3...
An upper-level trough negatively tilted through the Northern
Rockies will support lingering snowfall from the Idaho/Montana
border to northern Colorado through Wednesday. The greatest
snowfall accumulations for this period are expected ahead of a
series of western cold fronts within the Clearwater region of
Idaho, along the Bitteroot Mountains, and the Front Range of the
Rockies. After a dry period on Day 2, an incoming moisture-rich
Pacific system is forecast to bring additional snow to parts of
the Pacific Northwest. Warm air advection ahead of the system will
generally restrict snowfall to elevations above 3000 ft,
therefore, the Olympics and Washington Cascades will likely
receive the greatest accumulations, with WPC probabilities for at
least 4 inches of snow around 30-50 percent in the central
Washington Cascades. As the Pacific moisture is ushered further
inland by a mid to upper-level shortwave, snowfall will spread
into the higher elevation Northern Rockies. Moderate accumulations
are expected in the Idaho Panhandle, Bitteroot Mountains, and the
Teton Range. For the latter area, Day 3 WPC probabilities for at
least 4 inches of snow are around 50-70 percent.

...New England...
Day 2...
A deep, mature low pressure system is forecast to continue moving
east through southern Canada before lifting north into the Hudson
Bay early Thursday morning. Cold air and low to mid-level ascent
will allow for light snowfall accumulations from northern Vermont
through interior Maine. Most notably, passage of an attendant warm
front through New England will cause a rapid warming of the air as
precipitation continues throughout the region. This abrupt
temperature change will promote the development of freezing rain
from northern New Hampshire to north-central interior Maine. WPC
probabilities for freezing rain accumulations greater than 0.1
inch are about 20-30 percent from the northern White Mountains to
the Maine North Woods.

...Great Lakes...
Days 2 and 3...
The aforementioned cyclone moving through southern Canada is
forecast to pass to the northeast of the Great Lakes Thursday,
dragging cold Canadian air southward across the warm lake waters.
Interacting with surface shortwave instability and and a
moderately favorable atmospheric thermodynamic profile, these
primed northwesterlies will support the development of a few
multi-band snows off Lake Superior into the U.P. of Michigan and
downwind of Lakes Erie and Ontario into the Tug Hill Plateau and
western Southern Tier of New York. Several inches of snow will be
possible in each of these areas, where WPC probabilities for at
least 4 inches of snow are only about 10 percent. Elsewhere
downwind of the Great Lakes only light accumulations are expected.

For Days 1-3, the probability of significant icing > 0.25" is less
than 10 percent.

Zavadoff