Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
246 PM EST Tue Dec 17 2024

Valid 00Z Wed Dec 18 2024 - 00Z Sat Dec 21 2024


...Pacific Northwest through the Northern Rockies...
Day 1...

Shortwave ridging over the Pacific Northwest will be quickly
displaced by an approaching trough moving eastward from the Pacific
leading to height falls along the coast. While the core of this
feature will lift into British Columbia, a secondary vorticity
maximum will pivot onshore NW WA state tonight with enhanced ascent
through PVA, height falls, and concurrent upper level diffluence.
This impulse will move quickly eastward, but will be accompanied by
a lead warm front and trailing cold front, causing fluctuations in
snow levels.

The heaviest precipitation is likely to accompany the warm front,
generally from now through 12Z Wednesday, with a secondary surge
along the trailing cold front Wednesday morning. During the warm
frontal passage and accompanying WAA/moisture surge, snow levels
are progged to climb to as high as 8000 ft along and west of the
Cascades. This will limit significant snowfall to just the higher
terrain, with rain the primary p-type at the passes. However,
during the transition from cold to warm, a period of freezing rain
is possible even as low as Snoqualmie and Stevens Passes, creating
hazardous travel this evening before changing to all-rain. East of
the Crest, cold air locked in within Canadian high pressure will
allow for more substantial freezing rain/ice accretion through
early Wednesday. The subsequent cold front will produce primarily
rain, as snow levels crash behind it but occur with rapid drying as
well.

For the areas that get snow and ice accumulations, moderate impacts
are expected as reflected by the WSSI-P showing a high chance
(>80%) for moderate impacts in the WA Cascades and into the
foothills. Here, WPC probabilities for 0.1 inches of ice are
generally 10-30%, and 70-90+% for 6 or more inches of snow.


...Great Lakes into the Northeast...
Days 1-2...

A digging trough will emerge out of Canada and dive SW into the
Midwest and Great Lakes Wednesday. It will tap into a modest plume
of Gulf moisture moving northeast up the Ohio Valley, being driven
by a separate southern stream shortwave. While the primary surface
low will track across the Mid-Atlantic and up the New England
coast and into the Canadian Maritimes, the associated area of snow
will remain well north and west of the low center, starting across
much of Michigan Wednesday morning, then intensifying over New York
and into New England Wednesday night. The aforementioned northern
stream shortwave will translate eastward along the Canadian border
with the Northeast, helping to tap the moisture plume and cause
mostly advisory level snows on the order of 3 to 6 inches for most
areas from western New York through Maine. Lower elevations with
warmer temperatures will see less snow, with higher elevations
seeing a bit more. The snow will rapidly end from west to east late
Wednesday night and Thursday morning.

...Northern Plains to the Midwest...
Days 1-3...

A potent shortwave dropping from the Canadian Rockies and into
North Dakota by Thursday morning will support weak cyclogenesis in
the form of a clipper low with the primary ascent efficiently
overlapped with the LFQ of a strengthening jet streak also digging
into the region. This deep layer ascent will impinge into a low-
level baroclinic boundary as a warm front drapes eastward,
resulting in rapid cyclogenesis in eastern MT, with this low then
moving into ND and then into the Great Lakes by the daylight hours
on D2/Thursday.

As this wave moves eastward and deepens, WAA/isentropic ascent will
begin to intensify between 280K-285K on Thursday surging moisture
into and downstream of the system, reflected by an axis of PWs
exceeding the 90th percentile according to NAEFS. At the same time,
this WAA will occur favorably into an extremely cold column to
deepen the DGZ to more than 100mb, and intensifying fgen will drive
pronounced ascent into this DGZ. The deep DGZ, a sub-DGZ isothermal
layer, and modest winds in a very cold column suggest SLRs will be
quite high, and this will rapidly accumulate despite the general
progressive nature of this system. At this time, WPC probabilities
D2 are high (70-90%) for more than 4 inches across much of northern
ND, with a stripe of 30-50% probabilities for 4+ inches extending
as far east as the Door Peninsula of WI. Locally, 6-8" of snow is
possible across ND during the event.

Once the low gets to the Midwest Thursday evening, the greatest
forcing will shift south as the northern portion of upper level
energy supporting the precipitation shield favoring the snow over
the Great Lakes shears apart and rapidly weakens. Thus, by the day
Friday, much of the lower Great Lakes (outside of the lake-effect
corridors), Appalachians, and the NY/PA border counties should only
see flurries or very light snow. Arctic high pressure quickly
building in behind this clipper will dry the atmosphere out,
resulting in very little lingering snow except for any light lake-
effect.


Wegman/Weiss