Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
233 PM EST Sat Dec 7 2024

Valid 12Z Sat Dec 07 2024 - 12Z Tue Dec 10 2024

...Northeast...
Day 1 & Day 3...

Some additional lake enhancement is possible for a few more hours
downwind of Lake Ontario into the Tug Hill through mid-afternoon.
A storm system passing through the northern Great Lakes will track
quickly through southern Ontario tonight and into the Northeast by
early Sunday morning. With WAA and isentropic ascent being the
primary sources of lift, along with a progressive plume of
850-700mb moisture, snow will spread across the Northeast this
afternoon and conclude by Sunday morning as the storm heads for
Nova Scotia. The warm nose aloft caused by the low-level WAA may
lead to light ice accumulations in parts of the the interior
Northeast tonight and into Sunday morning. With the mean wind flow
more out of the west, upslope into N-S terrain (the Green and White
Mountains most notably) will promote moderate amounts of snow with
lighter amounts in lower elevations (coastal and valleys). This
storm progressive nature will limit amounts, as evident by WPC
probabilities for at least 4 inches of snow are highest (>70%)
north of I-90 in NY (Tug Hill/Adirondacks) and into the Green and
White Mountains. It is in these higher elevations (above 2,000ft)
where WPC probabilities have moderate chances (40-60%) for snowfall
totals above 6 inches. Note that Downeast Maine could also see
some minor snow accumulations as WPC probs depict moderate chances
(40-60%) for above 4 inches of snowfall through midday Sunday.

The next winter storm to affect the region comes in the form of low
pressure tracking through the Southeast that is accompanied by a
Gulf of Mexico moisture plume. Weak high pressure is located over
Quebec that will aid in the development of a cold-air-damming
signature as 850-700mb WAA out of the Southeast and subsequent
isentropic glide leads to snow being the primary precipitation type
over New England at the onset Monday afternoon. However, as WAA
increases, the burgeoning >0C warm nose aloft will force p-types to
change to a wintry mix in parts of central New England. This too
will be a progressive storm system, so snowfall totals are not
expected to be overly heavy. That said, WPC PWPF shows low chances
(10-30%) for minor snowfall accumulations of 2 inches or more in
parts of northern Maine on south to the Green and White Mountains.
These areas could also see minor ice accumulations Monday night
into Tuesday morning as well that could result in slick travel
conditions in parts of northern New England on Tuesday morning.


...Pacific Northwest/Northern Rockies...
Days 1-2...

The first of two shortwave troughs approaching the Pacific
Northwest is producing periods of high elevation mountain snow at
the moment, but snow levels will plummet to as low as 3,000ft later
tonight. Rain will change to snow at Stevens/Snoqualmie Passes
later this afternoon but snowfall rates should gradually diminish
at pass level by this evening. East of the Cascades, lingering
sub-freezing temperatures within the boundary layer are initially
trapped in the Columbia Basin as WAA and overrunning moisture aloft
provide a favorable setup for an icy wintry mix in central and
northeastern Washington today and even into parts of southeastern
OR/southwestern ID through this evening. Some minor icing
accumulations are possible up to about a tenth of an inch.

The second shortwave trough arrives tonight with periods of
mountain snow lingering into Sunday. Lower snow levels, compared
to the start of the first shortwave trough's approach on Saturday,
support moderate snow at pass level (especially Stevens Pass) with
this second wave of moisture. WPC probabilities sport high chances
(>70%) for at least 8 inches of snow above about 3,500ft while the
more remote areas above 5,000ft likely receive over 12" of
snowfall through Sunday evening. Snow should taper off by Sunday
afternoon as high pressure builds in from the West, thus allowing
for a more tranquil start to the work-week in the Pacific
Northwest.

Both atmospheric disturbances will push their respective plumes of
moisture east into the Northern Rockies with the heaviest snow
tonight and into Sunday as snow levels decrease, SLRs increase, and
northerly low level winds aid in upslope enhancement for snowfall
rates. Highest probabilities for >8" of snow are over the higher
terrain
including the Bitterroots, Lewis Range, Blue Mountains, and into
the Little Belt Mountains in central Montana, the Absarokas in
southern Montana, and the Tetons of western Wyoming.


...Northern Minnesota and northeastern North Dakota...
Days 1-2...

The first upper level shortwave trough that brought snow to parts
of the Pacific Northwest mountain ranges on Saturday will spawn
low pressure in the Canadian Prairies Saturday night. As its warm
front moves east, WAA across the Northern Plains will produce a
shield of precipitation that passes over a sub-freezing boundary
layer. While precipitation may start out as snow along the Canadian
border, mid/low-level WAA may promote an area of freezing rain for
portions of eastern North Dakota into Minnesota late tonight into
early Sunday. WPC probabilities do show some low chance
probabilities (10-30%) for >0.1" of ice accumulations in parts of
the Red River of the North and north-central Minnesota early Sunday
morning. In terms of snowfall, latest guidance supports the
heaviest snow occurring with the initial WAA over northern
Minnesota, and more specifically in the Minnesota Arrowhead. WPC
probabilities for at least 4 inches of snow are >50% from far
northeast North Dakota to much of northern Minnesota. The Minnesota
Arrowhead has moderate-to-high chances (50-70%) for >6" of
snowfall through Sunday evening. The WSSI-P shows moderate
probabilities (40-60%) for Minor Impacts (or winter driving
conditions that suggests some hazardous travel conditions) in far
northern Minnesota.


...CO Rockies/Sangre de Cristos...
Days 2-3...

Height falls associated with an elongated 500mb vorticity max and
a plume of residual Pacific moisture connected with the second
wave through the Pacific Northwest will push through the central
Rockies on Monday. High pressure building in from the north in wake
of a cold frontal passage Monday morning will result in some weak
upslope flow into Colorado's Front Range during the day. This
upslope component will extend as far south as the Sangre De Cristo
in northern New Mexico by Monday night. Periods of light-to-
moderate snow across the region on will ensue Monday into early
Tuesday. The progressive movement of the upper level disturbance
combined with the aforementioned atmospheric moisture plume being
gradually drying out through Tuesday morning will limit snowfall
totals. WPC probabilities show moderate probabilities (40-60%) of
at least 4 inches mainly limited to Sangre de Cristos in southern
Colorado and northern New Mexico, which does include the Raton
Mesa. The highest and more remote elevations could see localized
amounts approach 8" by the time the snow comes to an end Tuesday
morning.


The probability of significant ice >0.25" across the CONUS is less
than 10 percent.

Mullinax