Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
343 AM EST Sat Dec 18 2021

Valid 12Z Sat Dec 18 2021 - 12Z Tue Dec 21 2021

...Northeast...
Days 1-2...

Mid-level shortwave over the Upper Midwest this morning will move
through the Great Lakes today and the St. Lawrence River Valley
tonight in progressive flow. A wavy frontal boundary astride the
Ohio River will carry a surface low to the northeast today into
eastern OH/western PA before turning eastward just south of New
England. This will spread warm advection precipitation to the
Northeast states into a marginally cold column along the NY/PA
border eastward but a sufficiently colder air mass across northern
NY into central/northern New England where all snow is forecast.
Tail-end of a 130kt jet will provide broad-scale lift for the
region as a surge of moisture tied back to the Gulf (PWs around
0.75" or +2 sigma) will support a period of moderate snow today
into Sunday morning. Here, QPF will maximize around 0.60-0.80" as
a mid-level frontogenesis band pushes through. To the south,
milder air will push northward ahead of the surface low and cold
front, changing any sleet/freezing rain to rain before ending.
Models have slowly focused the area for best icing to northern PA
into the Southern Tier of NY eastward to the Catskills and
Berkshires where colder air will be slowest to erode.

WPC probabilities for at least 0.1" ice are 10 to 50 percent along
the PA/NY border across the Catskills and to the Berkshires. Snow
probabilities for 6 or more inches are moderately high (50 to 80
percent) from the Adirondacks eastward through the Green and White
Mtns through western Maine with lower probabilities over
southern/eastern Maine ending Sunday evening.


...Northwest U.S....
Days 1-3...

Compact closed low near Haida Gwaii will slide ESE and open up
across Vancouver Island today. 100+kt jet streak downstream of the
trough axis will pass across the Washington Cascades today,
providing broad-scale ascent as a warm front lifts through,
raising snow levels to around 4000-5000ft. The attendant cold
front will be quick to push into the area, with a modest plume of
moisture nosing into the area (PW values around 0.75" or about +1
to +1.5 sigma). Moderate snowfall is expected for mainly at and
above pass level with some freezing rain possible on the eastern
side of the Cascades where cold air may remain in place in some
sheltered valleys. A trailing upper shortwave will close off into
a low well off the OR/CA coast early Sunday and act to slow/stall
the cold front across Oregon, allowing a steady feed of moisture
into the Pac NW. This low pressure system then merely drifts east
off the OR/CA coast Monday which pushes the frontal boundary north
into WA while the brunt of the QPF sinks southward into northern
California by Monday. However, enough easterly flow into the
eastern slopes of the Cascades may support appreciable snow and/or
freezing rain post-FROPA in the lower elevations.

Day 1 WPC probabilities are high for 6 or more inches of snow over
the WA Cascades and into some passes. Probabilities expand to the
east over the northern tip of ID and northwest MT ranges where 1-2
ft is possible. By day 2, focus shifts southward into the OR
Cascades and across the Bitterroots. Probabilities lower by day 3
with only higher elevations showing 20-50 percent probabilities of
at least 6 inches of snow, but that includes parts of northern
California Shasta Siskiyous and Klamath Mountains.


For days 1-3, the probability of ice accumulation greater than
0.25" is less than 10 percent.

Fracasso