Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
311 PM EST Mon Dec 06 2021

Valid 00Z Tue Dec 07 2021 - 00Z Fri Dec 10 2021


...Great Lakes...
Days 1-2...

Brisk cyclonic flow on the backside of a strong low pressure
system in southeast Canada continues to introduce cold air
advection over the Great Lakes. Low level flow out of the W-WNW is
set to induce LES bands from Michigan's UP on east to northwest
Michigan and downwind of Lakes Erie and Ontario. Latest Day 1 WPC
probabilities show moderate chances of snow totals greater than or
equal to 4 inches positioned from the Porcupine mountains to as
far east as Sault Ste. Marie. Lower probabilities are present in
northwest Michigan and portions of northeast Ohio, northwest
Pennsylvania, far western New York, and downwind of Lake Ontario.
Given the strong westerly wind orientation over Lake Ontario
tonight, there could be totals that come closer to 6 inches near
the Tug Hill Plateau. LES bands are forecast to diminish on
Tuesday as the storm system to the northeast pulls away and high
pressure builds in from the west, but not before they drop a
couple more inches in favored locations downwind of Lakes
Superior, Erie, and Ontario Tuesday morning.

...Mid-Atlantic through Northeast...
Day 2-3...

An upper level feature rounding a low centered near James Bay
tracks east from the Ohio Valley Wednesday morning to off the
Northeast coast Wednesday evening. A quick moving disturbance,
this feature does produce PVA over the northern Mid-Atlantic along
with the favorable left exit region of a 250mb jet streak
positioned overhead early Wednesday morning. Latest guidance is
suggesting modest 700-500mb frontogenetical forcing aloft as well
as a moistening upper level profile to support snow in these
areas. However, the low-mid levels of the atmosphere start out
quite dry within the surface-700mb layer Wednesday morning and
there remains uncertainty in when/where the upper trough sharpens
up near the coast. Accumulations on average will likely range
between a coating to an inch in the lower elevations from the
Virginia on north up the I-95 corridor, although slightly higher
totals are possible where the best frontogentical forcing takes
shape. The highest totals that could approach 4 inches are
currently expected in the central Appalachians. By Wednesday
evening, the strengthening upper trough providing increased ascent
within the column in southern New England could lead to periods of
snow with light accumulations. Will be keeping a close eye on the
developing coastal low as some guidance members have shown the
potential for a TROWAL to produce several inches of snow along the
coast of Maine on Day 3 (Thursday). Latest Day 3 WPC snow
probabilities indicate 20-30% chance of snow totals greater than
or equal to 4 inches from Portland to Downeast Maine. 

...Pacific Northwest to the Colorado Rockies...
Days 1-3...

Much of the period can be summarized as a parade of Pacific
disturbances tracking into western North America that end up
crossing the Intermountain West through Thursday. The first is set
to track through central Montana with snow likely in the
Bitterroots, Absarokas, and Tetons tonight. Meanwhile, a sheared
off portion of the upper trough just west of California will
supply some moisture to the Great Basin and central Rockies. The
latter of which becomes positioned under the right entrance region
of a 130+ knot jet streak centered in the Midwest. These factors
support the opportunity for snow totals in excess of 6 inches in
the northern Colorado Rockies Monday night into Tuesday morning.

The next shortwave trough and accompanying Pacific jet streak
reaches the Northwest Tuesday night with a steady barrage of
upslope flow into the Olympics and Cascades, producing periods of
higher elevation snow from Wednesday into early Thursday.
Meanwhile, moisture streaming ahead of the upper trough will
result in more mountain snow for the Bitterroots, Lewis Range, and
Tetons. Broad scale upper level divergence ahead of the trough and
upslope flow favors developing snow over the Sierra as snow levels
drop Wednesday night. As a trailing vort max dives south off the
coast of British Columbia Thursday morning, it will further
enhance diffluent flow aloft and lead to a developing area of low
pressure in the Intermountain West the second half of the day.
Periods of moderate-to-heavy snow are then likely to breakout from
the central Great Basin to the Wasatch and Colorado Rockies. This
strengthening storm system Thursday evening lays the groundwork
for a potential winter storm in the central High Plains come
Friday.


For Days 1-3, the probability of ice accretion over 0.1" is less
than 10 percent.

Mullinax