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

Valid 12Z Tue Dec 21 2021 - 12Z Fri Dec 24 2021

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

Active period in the West for at least the next three days as an
upper low off California weakens only to be replaced by another
from the Gulf of Alaska. Leftover moisture plume along the OR/WA
coast to start day 1 will get squeezed inland ahead of the
incoming Gulf of Alaska system tonight, with the heavier snow over
the Washington Cascades. By day 2, jet starts cranking out of the
subtropics (150-160kts) across the northern Sierra and southwest
flow will draw in an ample supply of moisture as PW values rise to
around 0.75" or +1 to +2 sigma (and higher to the southwest near
the coast). This modest AR will slowly sink southeastward through
day 3 ahead of the surface frontal boundary to the northwest. Snow
levels will increase through day 1 ahead of the trough, then start
to decrease by the end of day 2 as colder air starts to push
southeastward. On day 3, snow levels are forecast to drop below
3000-4000 ft over northern CA and to around 5000ft over the
southern Sierra. Snow accumulation will pick up on day 3 over the
Sierra where more than 2 feet is expected amid intense snowfall
rates. QPF shield will push through the Great Basin and into the
central Rockies with snow picking up in earnest over the Wasatch.
To the north, incoming northern jet into Washington will enhance
snowfall again over the Cascades with low snow levels by day 3
around 1000-2000ft.

For California, WPC probabilities of at least 12 inches of snow
will increase from day 2 to day 3 (50-70 percent to 70-95 percent)
in higher elevations. Probabilities of at least 4 inches of snow
will increase as well but also at lower elevations, where areas
above 4000ft have a 30 percent chance. To the north, WPC
probabilities are moderate to high over the Cascades on day 1,
then increase by day 2 to over 70 percent into the Oregon Cascades
and the Sawtooth of ID. By day 3, large area of the mountainous
West will see at least a moderate chance of several inches of
snow, including into the Wind Rivers/Tetons of WY and the Park
Range, Sawatch Range, and San Juans in CO.


...Northern Tier/Great Lakes...
Day 1...

Quick-moving shortwave and surface low will race through the Upper
Midwest and Great Lakes today with a brief period of snow from MN
eastward in warm air advection. Cold column and modest lift
through the DGZ will promote SLRs nearing 20:1 but be limited in
duration. As the surface low passes through or just south of the
U.P., flow across Superior will yield a bit of lake effect over
the western U.P. where more than 4 inches is likely (WPC
probabilities of at least 4 inches are 50-80 percent there as well
as over the eastern U.P.).


...New England...
Day 2...

Low pressure in the Gulf of Mexico will translate along the
frontal boundary into the western Atlantic and lift northeastward
outside the 40/70 benchmark Wednesday as it deepens into the 980s
mb. Shortwave out of the Great Lakes will eventually wrap into and
overtake the coastal system well into Atlantic Canada, but not
before interacting enough to spread a wintry mix over much of New
England. Though the guidance still shows disagreement in the
evolution/details and track, rapid deepening should allow
sufficient moisture to wrap back into eastern Maine along a
surface trough to yield several inches of snow. Brief overlap of
LFQ of the southern jet and RRQ of the northern jet at the start
of day 2 should enhance broad-scale lift even as it exits the
region as WAA pushes through the area. WPC Snowband Probability
Tracker shows several CAMs simulating 1-1.5"/hr rates over eastern
Maine Wednesday afternoon. Some freezing rain is also possible
just away from the coast where surface temperatures may be
sub-freezing as enough of a warm nose nudges into the area. WPC
probabilities of at least 4 inches of snow are highest over
northern and eastern Maine away from the coast (above 70 percent).


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

Fracasso