Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
340 PM EST Tue Dec 21 2021

Valid 00Z Wed Dec 22 2021 - 00Z Sat Dec 25 2021

...Western U.S...
Days 1-3...

An extremely active period of winter weather is expected across
the West through the weekend. A mid-level closed low off the
Pacific coast will be replaced by subsequent pieces of energy
diving out of the Gulf of Alaska, while the lead energy shifts
onshore the Pacific coast. This evolution will maintain mid-level
divergence, repeated height falls, and pronounced moisture
advection as IVT surges to as high as +4 sigma (on D3, NAEFS
ensemble tables) and surges southward through the period.
Continuous moist advection will maintain waves of precipitation
across the region, and while the heaviest QPF is likely focused
across the Sierra due to orthogonal flow efficiently upsloping and
within the longest duration of max IVT, heavy precipitation is
likely across the entirety of the Intermountain West. Snow levels
initially will be 3000-5000 ft across the north but as high as
8000 ft in CA, but will gradually lower, especially late D2 and D3
when the trough and accompanying cold front push snow levels to
1000-2000 ft north of CA, and 3000-6000 ft further south.

Snow is likely to be widespread above these snow levels each day,
but the highest accumulations D1 are likely in the WA Cascades and
Olympics where WPC probabilities for 12+ inches are high. 6 inches
or more is likely on D1 as far east as the Northern Rockies,
Sawtooth, and Wind Rivers, as well as into the Sierra. However,
much heavier snow is likely in the Sierra beginning D2, above 7000
ft, as the atmospheric river strengthens and focuses into that
range. WPC probabilities D2 are high for 18 inches, and locally
more than 4 feet is forecast. Heavy snow noted by WPC
probabilities of more than 50% for 12 inches on D2 are also
present along the entirety of the OR and WA Cascades, the northern
CA ranges, and into the Sawtooth of ID and Wind Rivers/Tetons of
WY. On D3, anomalous moisture will be present across nearly all of
the Intermountain West, and with robust forcing in place, heavy
snow is likely to accumulate above 12 inches according to WPC
probabilities from the Olympics eastward to the Northern Rockies,
south into the San Juans and Wasatch, and again in the Sierra
where an additional 2-3 ft is possible. With the lowering snow
levels, significant snowfall is also likely at many of the
mountain passes, with more than 12 inches of snowfall possible at
Stevens, Snoqualmie, and Santiam Passes D3.


...Great Lakes...
Day 1...

Fast moving wave of low pressure moving east of the Great Lakes
will leave robust CAA in its wake with nearly unidirectional flow
from the N/NW across the Lakes. 850mb temps crash towards -15C,
producing steep lapse rates and pushing an inversion up towards
8000 ft. With the most robust ascent focused in the lowering DGZ,
lake enhanced snowfall is likely in the favored N/NW snow belts,
especially across the U.P. of MI and the northern L.P. near
Traverse City. SLR's approaching 20:1 in the cold column will
produce significant snowfall despite limited QPF, and WPC
probabilities for 4 inches or more are above 50% in the eastern
U.P. around Whitefish Bay, with locally in excess of 6 inches
possible. Otherwise, WPC probabilities for 2 inches are high in
the NW L.P. of MI, and moderate along the Tug Hill Plateau and the
Chautauqua Ridge.


...New England...
Days 1-2...

Low pressure moving up the Atlantic coast will strengthen rapidly
in response to impressive diffluence within a coupled jet
structure and height falls ahead of a rapidly approaching
shortwave moving across the Great Lakes towards New England. The
interaction of these features should lead to a surface low moving
near the benchmark while deepening into the 980s. The interaction
of the approaching shortwave with this primary low will lead to an
expansion of the precipitation shield to the NW on increase
isentropic ascent and WAA, and a period of moderate to heavy
wintry precipitation is likely from far eastern New York state
through Maine. The guidance has trended a bit warmer overall once
again, so while the surface high retreats slowly, persistent dry
isallobaric surface drainage will promote wet-bulb temperatures
below 0C at least until the WAA overwhelms the environment, and a
corridor of freezing rain is likely from the Mohawk Valley of NY
through NW CT, western and central MA, into southern NH and much
of coastal ME. WPC probabilities in this region are 5-10% for 0.1"
of accretion. To the north, primarily across Maine, a period of
heavy snowfall with rates of 1"/hr or more is likely Wednesday
aftn and night within the best deformation axis. WPC probabilities
for 6 inches of snowfall are above 70% from the Central Highlands
northeast to the Canadian border, with local storm total of 10
inches possible near Caribou.


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

Weiss