Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
229 PM EST Mon Nov 27 2023

Valid 00Z Tue Nov 28 2023 - 00Z Fri Dec 01 2023

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

--A significant lake effect snow (LES) event begins today and
becomes intense tonight into Tuesday. Shifting winds and somewhat
drier air will result in a weakening of LES on Wednesday.--

Cyclonic flow over the Great Lakes that is simultaneously ushering
in 850mb temps as cold as -15C aloft will race over the mild Great
Lakes today and spark potent lake effect snow showers and bands
through Tuesday. The combination of -10C to -15C 850mb temps
racing over lake temperatures that range from +5 to +12C are a
recipe for exceptionally steep low level lapse rates. In fact, as
much as 500-1000 J/kg SBCAPE will be available at some of these
lake effect bands disposal, particularly downwind of Lakes Erie
and Ontario. Sampled soundings near Erie, PA this afternoon and
downwind of Lake Ontario this evening show exceptional vertical
velocities in the 925-800mb layer that are both highly saturated
and located within the DGZ aloft. These soundings suggest snow
bands could produce 2-3"/hr snowfall rates at times tonight, as
well as instances of thundersnow within the more intense bands.
Add in 12-24 hours worth of lifespan for these bands over Michigan
U.P., western and northern Michigan, and from northeast
OH/northwest PA to the areas of western and northern NY downwind
of Lakes Erie/Ontario, and the stage is set for heavy snowfall.

The WSSI is suggesting Major Impacts are anticipated from the
Cleveland metro area on north and east along the Chautauqua Ridge.
Major impacts are also expected on the Tug Hill Plateau through
Tuesday night. In these areas, residents can expect considerable
disruptions to daily life that include the potential for
widespread closures and dangerous to impossible travel. WPC 48-hr
probabilities for >18" suggests a high chance (80-90%) for >18" of
snowfall on the Tug Hill Plateau through Wednesday morning. Along
the The Chautauqua Ridge, WPC 48-hour probabilities for >12" of
snow are also showing a high risk (70-90%) for >12" of snowfall.
The immediate Cleveland metro areas shows a moderate-to-high risk
(50-70%) for >12" of snow. Lake effect bands will meander at times
tonight into Tuesday morning, but begin to shift to a more WNW-ESE
orientation Tuesday afternoon. This could lead to rounds of heavy
snow in areas such as Syracuse and slightly farther inland from
the Lake Erie/Ontario shorelines.

Farther north and west, lake effect streamers will focus
themselves over the eastern half of Michigan's U.P. and over the
northwestern portion Michigan. WPC 24-hr probabilities show
moderate-to-high risks (60-80%) for >4" from just east of
Marquette to Sault Ste. Marie through Tuesday afternoon.
Meanwhile, a vorticity maximum revolving beneath the base of the
upper trough aloft will promote additional vertical ascent over
the Greens and Whites of VT/NH and could produce as much as 4" of
snow locally in some of these ranges tallest peaks on Tuesday.

...Southern Rockies...
Day 3...

An upper level trough tracking through the Southwest late
Wednesday will usher in a ribbon of Pacific moisture aloft while
sufficient upper level divergence ahead of the trough aids in
generating precipitation. Mean winds oriented out of the SW will
be oriented orthogonally to the San Juans in southern CO and
northern NM, leading to the potential for enhanced snowfall rates
due to topographically-induced upslope flow. Latest WPC 24-hr
probabilities show a low risk (10-20%) for snowfall totals >4" in
the San Juans with elevations >9,000ft most likely to see those
>4" totals. The WSSI-P does show as high as 30-40% odds of
witnessing Minor impacts in the San Juans on Thursday.

Mullinax