Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
329 AM EST Wed Dec 22 2021

Valid 12Z Wed Dec 22 2021 - 12Z 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 (and beyond this forecast period --
see PMDEPD for more info on the medium range). A sharpening
shortwave diving out of the Gulf of Alaska will split NE-SW
into/through the West today/tomorrow before yet another shortwave
rides around the seemingly permanent central Pacific upper ridge
on Friday into the Pac NW. This evolution will maintain mid-level
divergence, repeated height falls, and pronounced moisture
advection as IVT surges to as high as +4 sigma/99th percentile
(D2-3, 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. Initial surge into the Sierra will push
through the Great Basin to the Rockies into Friday when the next
system comes toward WA/OR with another surge in moisture for the
Coastal Range and Oregon Cascades with lowering snow levels.
Initially, snow levels will be 3000-5000 ft across the north but
as high as 8000 ft in CA, but will gradually lower through the
next few days as colder air moves in and is reinforced. Snow
levels may drop toward lower elevations (not quite sea level) by
the end of the period over Washington to about 500-1000ft.

Snow is likely to be widespread above these snow levels each day,
with the highest accumulations D1 likely in the WA
Cascades/Olympics southward to the Sierra Nevada. Snowfall will
expand eastward through the northern and central Rockies D2
including the Great Basin southward to about the UT/AZ and CO/NM
line. On D3 the next surge reinvigorates the Northwest as snow
amounts lessen but don't abate over the Sierra and even eastward
to the CO Rockies. Multi-day probabilities of at least 18 inches
are high over the terrain where several feet are likely in the
Sierra and along the spine of the Cascades. 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 D2.

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

Low pressure off the Carolina coast will rapidly deepen and move
into Atlantic Canada as a coupled jet structure fosters
development. This will be aided by an incoming northern stream
shortwave from the eastern Great Lakes this morning. The
interaction of the approaching shortwave with the primary low,
though outside the benchmark, will lead to an expansion of the
precipitation shield to the NW on increased isentropic ascent and
WAA. A period of moderate to heavy wintry precipitation is likely
over eastern New England and especially northern/eastern (but
inland) Maine. With some rain along the coast in a warmer
environment and snow inland over the higher elevations, a mix of
sleet and freezing rain is likely in between along and inland from
I-95 up to about 0.1" -- though some guidance was higher. To the
north, a period of heavy snowfall with rates of 1"/hr or more is
likely this afternoon and evening 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 totals of 10 inches possible near Caribou.


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

Fracasso