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

Valid 12Z Mon Nov 27 2023 - 12Z Thu Nov 30 2023

...Great Lakes...
Days 1-3...

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.

Expansive cyclonic flow will develop across the eastern 2/3 of the
CONUS today around a large gyre centered south of the Hudson Bay.
Around this feature, spokes of vorticity will shed south and east
through the Great Lakes, providing reinforcing shots of cold air
and a secondary front Monday night behind the primary front which
just shifted east of the area. Each of these fronts will cause
renewed CAA with 850mb temps progged to crash to around -15C. This
cold air will move atop the still very warm lakes noted by GLERL
lake-surface temperatures of +5 to +12C, producing steep lapse
rates and deepening inversion depths above 15 kft at times to
support SBCAPE around 1000 J/kg. The steep lapse rates and
impressive ascent within this increasingly favorable environment
will support intense LES bands, especially downwind of Lakes Erie
and Ontario where shear direction will favorable move along the
longer fetch of these lakes. Additionally, an upstream connection
to produce an even longer effective fetch is likely, and as this
occurs in conjunction with some additional synoptic moisture
Monday night into Tuesday beneath one of the vorticity lobes,
snowfall rates will become extremely robust. The WPC prototype
snow band tool, which admittedly struggles to detect LES bands due
to their small size, has a clear signal for 2-3"/hr snowfall
rates, which is further evidence of the potential intensity of
this event. It is not out of the question that lightning and
thunder will accompany the LES at times.

The heaviest snow is likely D1 into D2, before brief shortwave
ridging ahead of the next trough Wednesday causes less favorable
flow with some weak WAA. At the very least, this evolution by D3
will result in the snow bands becoming more oriented SW to NE,
pivoting away from the areas that are expected to receive heavy
snow D1-2 east of Lakes Erie and Ontario. Additionally, this
should cause snow off the other lakes to pivot into Canada or shut
off entirely, at least briefly, before renewed shortwave troughing
late D3 causes renewed, but likely less intense, LES into Thursday
morning.

The heaviest snow is expected downwind of Lakes Erie and Ontario,
but the maximum axes should remain south of the Buffalo and
Watertown metro areas. Here, WPC probabilities D1 are 40-70% for
12+ inches, and remain above 70% D2 into the Tug Hill plateau
while waning over the Chautauqua Ridge. By D3 the greatest risk
for heavy LES pivots into the Watertown area E/NE of Lake Ontario,
where WPC probabilities for more than 6 inches rise to 30-50%.
Storm total snowfall will likely reach 1-2 feet with isolated
higher totals, and potentially as much as 4 feet in the Tug Hill
plateau, although some uncertainty into the persistence of these
bands at any give location could significantly alter the snowfall
amounts. Regardless, an impactful LES event is expected to begin
later today downwind of these lakes, with major impacts possible
as reflected by the WSSI.

Farther west, heavy LES is likely across the U.P. and northern
part of the L.P. near Traverse City as NW flow and the associated
CAA move across the warm waters of Lake Superior and Lake
Michigan. While light snow is expected in these same areas D2 and
D3, the most significant accumulations are expected on D1 when WPC
probabilities for more than 6 inches reach 30-60%, and locally
more than 12 inches are possible in the eastern U.P.

Weiss