Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
324 AM EST Sun Dec 18 2022

Valid 12Z Sun Dec 18 2022 - 12Z Wed Dec 21 2022

...Maine...
Day 1...
A deep, slow-moving, closed low continues to churn over the Upper
Great Lakes and southern Ontario this morning, with an elongated
trough extending to the east over northern New England. Light to
moderate snow is expected over much of Maine on Sunday as the
upper-level low eventually swings over the region and re-energizes
a surface low over Nova Scotia on Monday. Favorable upper forcing
and moisture in additional to enhanced low level convergence will
help support additional snowfall totals over 6 inches over central
and northern Maine, even though snowfall rates should generally
remain under 1"/hr. WPC PWPF shows widespread high probabilities
for additional accumulations of 4 inches or more from Interior
Downeast Maine on north, with embedded low probabilities for 8
inches in northern Maine. 

...Great Lakes...
Days 1-2...
The previously noted closed low over the Upper Great Lakes that is
forecast to gradually shift east and elongate will allow for cold
air to continue spreading across the eastern Great Lakes. This is
forecast to lead to additional heavy lake effect snow downwind of
lakes Erie and Ontario. Strong west-southwesterly winds will
create a ripe environment for persistent bands to impact the Tug
Hill Plateau and far western New York (just south of Buffalo)
through late Monday. By the start of the workweek, winds will
shift to more of a northwest direction in response to the exiting
upper level low and weaken, cutting off the lake effect snow
machines. Through Monday, additional amounts of a foot or more are
possible, with the Tug Hill likely to see the highest amounts
potentially up to 2 feet.

...Pacific Northwest & Northern Rockies...
Days 1-3...
Two separate shortwaves rounding a closed upper-level low spinning
over southwest Canada are expected to impact the Northwest and
northern Rockies over the next few days with potentially heavy
snow. Starting with today into early Monday, a strengthening 250
mb jet streak around 130 kts will help support enhanced orographic
lift throughout the northern Cascades and northern Rockies.
Specifically, continued light to moderate snowfall is possible
across northern Idaho and northwest Montana. WPC PWPF shows high
probabilities for greater than 6 inches of snow through Monday
across these regions, with lower probabilities for greater than 8
inches in the tallest mountain ranges. By Tuesday into early
Wednesday, a second and much stronger shortwave is expected to
slide southeastward over the Pacific Northwest as a strong arctic
cold front also sinks southward. This will allow for snow levels
to remain below 500 feet across much of Washington when
precipitation enters. The best QPF is likely to be confined to the
Cascades and Olympic Peninsula, but light amounts are also
expected into the Puget sound region. WPC PWPF depicts medium to
high probabilities for greater than 6 inches of snowfall on
Tuesday and Tuesday night across the Washington Cascades as well
as the Olympic Peninsula, with these probabilities also extending
to lowlands on the north and west side of the Olympics. Farther
downstream into the northern Rockies, additional heavy snow is
possible across parts of northern Idaho, the Bitterroots, and
northwest Montana. WPC PWPF shows medium (40-60 percent) chances
for greater than 8 inches, which equates to high 72 hour
probabilities for greater than a foot of snow.

The probability of receiving at least 0.10" of freezing rain is
less than 10 percent.

Snell