Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
350 PM EST Wed Nov 17 2021

Valid 00Z Thu Nov 18 2021 - 00Z Sun Nov 21 2021

...Pacific Northwest to the northern Rockies...
Days 1-3...

A Pacific system in the mid-latitudes will push into OR Thursday
night, with broad ascent via the northern stream jet across
British Columbia on Thursday. Snow levels will rise in modest warm
air advection with generally moderate late Thursday/Thursday night
over the Olympics and Washington Cascades where Day 2 WPC
probabilities are moderate for 6 or more inches. Through Friday,
the mid-level shortwave and the surface low will weaken as its
associated moisture continue inland into the Idaho panhandle,
Bitterroots, and the Teton Range as snow levels rise to about 5000
feet where there are moderate Day 2.5 snow probabilities for 6 or
more inches.

...Northern New England...
Day 1...

Snow/sleet/freezing rain and rain progress in tandem with warm
advection associated with a warm front lifting over northern New
England tonight. Areas in deeper cold air (northern Maine) will
see a period of accumulating snow before a changeover to sleet and
freezing rain then rain. Portions of the northern White Mountains
through the Allagash will see a period of ice accumulation up to
around a tenth of an inch overnight tonight as milder air moves
in.

...Great Lakes...
Days 1/2...

A deep low shifting east through Ontario tonight into Thursday
will usher colder air (around -10C at 850mb) across the relatively
warmer Great Lakes. With sufficient but modest low-level moisture,
multi-band snows are favored off Lake Superior into the U.P. and
across western Michigan Thursday, and downwind of Lakes Erie and
Ontario on WNW winds Thursday night/Friday. Day 1/1.5 snow
probabilities for 4 or more inches are the Keweenaw and northern
LP of MI with Day 2 probs limited around Chautauqua and the Tug
Hill in NY. 

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

Fracasso/Jackson