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

Valid 00Z Sun Dec 19 2021 - 00Z Wed Dec 22 2021

...Northeast...
Day 1...

A mid-level shortwave over the western Great Lakes this afternoon
will move through the eastern Great Lakes and the St. Lawrence
River Valley tonight in progressive flow.   Surface low pressure
exiting the Ohio Valley will continue east across Pennsylvania and
New Jersey during the evening hours, before tracking near Long
Island and the southeastern coast of New England during the
overnight.  This will spread warm advection precipitation into the
Northeast states into a marginally cold column along the New
York-Pennsylvania border eastward, but a sufficiently colder
airmass across northern New York into northern New England, where
mostly snow is forecast.  The 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 beginning this evening and continuing into Sunday
from the Adirondack region eastward into northern New England.  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.

While widespread significant ice accumulations are not expected
during the evening and overnight hours, areas of accumulating ice
can be expected across portions of eastern Upstate New York into
central and northern New England.   Within this area, WPC
probabilities indicate a greater potential for relatively heavier
accumulations across the Berkshires, with some 10-40 percent
probabilities for accumulations of 0.10 inch or more.   Moderate
to locally heavy snows are expected to extend east from the
Adirondacks into northern New England this evening and shifting
east across northern New England to Downeast Maine during the
overnight.  The heaviest evening and overnight accumulations are
expected to center from northern Vermont, northern New Hampshire,
and interior western Maine to Downeast Maine.  WPC probabilities
indicate that accumulations of 4-inches or more are likely across
this area.

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

A compact upper low dropping south near Vancouver Island today is
forecast to shear east across southwestern Canada overnight,
pushing the associated cold front east from Washington into the
northern Rockies and south into Oregon.  Accompanied by a modest
plume of moisture -- PW values around 0.75 inch or about +1 to
+1.5 sigma -- moderate snowfall is expected for mainly at and
above pass level.   A trailing upper shortwave will close off into
a low well off the Oregon/California coast early Sunday and act to
slow/stall the cold front across Oregon, allowing a steady feed of
moisture into the Pacific Northwest. This low pressure system then
merely drifts east off the Oregon/California coast Monday which
pushes the frontal boundary north into Washington. As the low
level flow become more parallel to the coast and then offshore,
precipitation is expected to diminish across Washington Monday
night into Tuesday.  Meanwhile, energy ejecting out ahead of the
upper low is expected to push a frontal boundary, accompanied by
deeper moisture into Northern California on Tuesday.  This is
expected to bring the next chance for locally heavy snows back
into the Klamath Mountains, the Shasta Cascade region, and the
northern Sierra Nevada.

For Day 1 -- ending 00Z Monday -- probabilities for accumulations
of 6 inches or more are high across portions of the southern
Washington and northern Oregon Cascades, as well as the Blue
Mountains and parts of the northern Rockies.  This includes a
large portion of northern Idaho and northwestern Montana,
including both Marias and Lookout passes.  On Day 2 -- ending 00Z
Tuesday -- the higher probabilities for accumulations of 6 inches
or more shift a little farther north along the Cascades, while
dropping off some farther east across the Blue Mountains and
northern Rockies.  By Day 3 -- ending 00Z Wednesday -- the high
probabilities for accumulations of 6 inches or more are confined
to the mountains of Northern California, including the Klamath
Mountains, the Shasta Cascade region, and the northern Sierra
Nevada.

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

Pereira/Fracasso