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

Valid 00Z Thu Dec 23 2021 - 00Z Sun Dec 26 2021

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

Continuously reinforcing longwave trough across the West will
drive an extremely active period of winter weather through the
forecast period and beyond (see PMDEPD for more information on the
medium range). As the primary trough axis repeatedly reloads just
off the Pacific Coast, spokes of vorticity will shed eastward
Thursday, Friday, and Saturday, across different latitudes along
the coast. While these shortwaves will produce synoptic ascent
through height falls and PVA, a persistent subtropical jet streak
will arc from the Pacific into the middle of the CONUS, combining
with warm and moist advection ahead of any surface fronts to
produce anomalous moisture noted by PW anomalies of +2 to +3
standard deviations above the climo mean. At the same time, snow
levels across the West will range from about 2000 ft near Canada
to as high as 8000 ft within the core of the moisture plume D1 and
D2, but will then fall quickly, especially by D3 as arctic air
funnels down from Canada. by D3, snow levels across WA/ID/MT will
drop to near sea level, with snow levels around 1000 ft in OR, and
as low as 4000 ft in Southern CA. This could produce light
snowfall even in the lowlands of the Pacific Northwest on D3.

Moderate to heavy snow is likely each day across portions of the
West, with snow spreading across nearly all the terrain from the
Olympics of WA southward into the Transverse ranges of Southern
CA, along the Mogollon Rim of AZ, eastward into the San Juans and
Sangre De Cristos, and northward to the Northern Rockies. 3-day
snowfall will likely exceed 6-7 feet in the Sierra, with more than
3 feet likely in the higher terrain of the Cascades, Olympics,
Sawtooth, Wasatch/Unitas, Tetons, and the highest peaks of the CO
Rockies and San Juans. Snowfall will likely occur in waves, but
WPC probabilities for 12 inches are moderate to high across nearly
all of these ranges each day of the forecast period. By D3 as the
snow levels crash, light snow will spread into the lowlands, and
WPC probabilities for 1" are 10-30% including the metro areas
around Portland and Pendleton, OR, as well as Seattle, WA. With
snow levels falling, nearly all of the mountain passes from the
Sierra northward will experience impactful accumulating snow
through the period.



...Northeast...
Day 3...

A strung out shortwave embedded within broad cyclonic flow across
the CONUS will lift east-northeast across the Plains into the
Great Lakes Saturday producing a weak wave of surface low pressure
beneath it. Ahead of this wave, a warm front will extend eastward
from this low and surge northeast towards New England late D3. As
this precipitation surges northward on a band of enhanced WAA, it
will spread precipitation into the area atop a weak high pressure
centered over Canada. This should produce a swath of freezing rain
as the warm nose lifts atop the persistent cold surface air, but
total precipitation is currently expected to be light. WPC
probabilities on D3 are as high as 20% for 0.1" of freezing rain,
focused in the Catskills and Adirondacks.

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

Weiss