Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
303 PM EST Sun Nov 26 2023

Valid 00Z Mon Nov 27 2023 - 00Z Thu Nov 30 2023

...Great Lakes and Northern New England...
Days 1-3...

Confidence continues to grow in a multi-day significant lake
effect snow (LES) event that looks to blanket quite a few areas
downwind of Lakes Superior, Michigan, Erie, and Ontario in
moderate-to-heavy snowfall over the next 2-3 days. There is also
likely to be heavy snowfall to discuss over portions of northern
New England.

Focusing on the snow in northern New England first, a surface low
forming out ahead of a sharpening 500mb vort max tracking over the
Northeast coast becomes the focus for a developing band of heavy
snow from the Adirondacks on north and east Sunday evening. The
heaviest bursts of snow are likely to occur from the Green and
White Mountains and into north Maine. It is in these parts of
northern New England where anomalous features, such as <10th
climatological percentile heights over the Great Lakes and >90th
climatological percentile IVT values surging north off the
Atlantic that will foster an environment suitable for falling
heights and adequate moisture aloft. As strong 850mb WAA and 290K
isentropic glide ensues tonight, snow will pickup over northern NY
and northern New England and, according to the 12Z HREF, features
a high risk (80-95%) for snowfall rates >1"/hr tonight and into
northern Maine on Monday. WPC probabilities have continued to
trend towards higher totals, highlighted by WPC probabilities for
>6" of snowfall topping out between 40-60% in the White Mountains
and as far north as Maine's North Woods. Expect hazardous travel
conditions on roads in there areas, most notably in northern Maine
where the WSSI shows Minor Impacts possible due to Snow Amount and
Blowing Snow.

In the Great Lakes, as the primary cold front swings through Lakes
Erie and Ontario this evening, a secondary cold front racing
through the Upper Great Lakes will inject a more frigid air-mass
over the Lakes Superior and Michigan by early Monday morning. This
will kick-start the LES machine downwind of these Lakes over the
Michigan U.P. and western Michigan. By 12Z, the secondary front is
set to race across Lakes Erie and Ontario and trigger a LES event
that will last into the middle of the work-week. As 850mb temps as
cold as -10 to -15C race over the lakes within mean layer flow
that is 265-270 on average, the expectation is for robust
single-banded segments to take shape. 12Z HRRR soundings within
the show textbook high omega, highly saturated, steep surface-3km
lapse rates at low levels that are often observed within lake
effect snow bands that can produce 2-3"/hr snowfall rates Monday
night and into Tuesday morning.

WPC 48-hour probabilities (encompassing Monday and Tuesday) are
keying in on the Tug Hill Plateau and the Chautauqua Ridge as
having the best chances of reeving over a foot of snow.
Probabilities in the Tug Hill for >12" have risen to a fairly
confident 90% chance, with even >18" probabilities as high as 80%.
Probabilities are they are a bit lower but still suggest a
moderate risk (40-50%) for >12" snowfall totals. along the
Chautauqua Ridge. The WSSI shows Moderate Impacts are forecast
from as far west as the Cleveland metro to the southern Buffalo
suburbs. Major Impacts could be felt in the Tug Hill and in
northwest PA/northeast OH. Snow Amount is the primary driver
forcing these impactful criteria on the WSSI, but note that the
WSSI-P also shows 40-50% probabilities for Snow Rate to cause
Minor Impacts. Overall, the combination of heavy snow, gusty
winds, and exceptional snowfall rates are likely to cause
significant travel delays downwind of Lakes Erie and Ontario
Monday evening and lasting into the day on Tuesday. Meanwhile, the
Michigan U.P. and northwest Michigan are showing moderate-to-high
risks (50-70%) for >6" in parts of these areas through Tuesday.
WSSI shows mainly Minor Impacts expected, although Moderate
impacts in areas where snowfall totals can approach or surpass 8"
are within the realm of possibility.

Mullinax