FOUS11 KWBC 300842
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
342 AM EST Sat Nov 30 2024

Valid 12Z Sat Nov 30 2024 - 12Z Tue Dec 03 2024

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

*** Heavy Lake-Effect Snow event into Tuesday ***

Broad cyclonic flow from the upper levels to the surface will
continue across eastern Canada and across the Great Lakes through
Tuesday. North to northwesterly flow across the Upper Lakes becomes
more westerly as the flow moves over the lower lakes. Lake SSTs are
well above average for this time of year, with GLERL reporting
water temperatures of anywhere from +8 to +15 degrees Celsius (46
to 59F). Meanwhile, air temperatures of the Arctic air mass moving
over the warm lakes is forecast to remain -10 Celsius or colder at
850 mb for the next several days. The difference in temperature is
much greater than the standard rule of thumb threshold of 13
Celsius...with the warmest lake temperatures at or double the
minimum threshold. This means that there is plentiful instability
to sustained continued lake-effect into next week. The sometimes
extreme instability will support cellular convection embedded
within well-formed lake-effect bands and may include thundersnow.

The cyclonic flow and continued moisture influx will also favor
broad atmospheric lift irrespective of the lakes. This favorable
atmosphere will maximize each lake's ability to continue
producing heavy lake-effect snow. The general northwesterly flow
will also allow for the upper lakes to feed the lake-effect off the
lower lakes. This will be a large contributor to the prodigious
snow totals forecast southeast of the lower lakes. Lake Superior
and Georgian Bay (part of Lake Huron) will feed into Lake Ontario's
lake band, while the bulk of Lake Huron will feed into Lake Erie's
lake-effect bands. Lake Erie will be increasingly dependent on 
Lake Huron's support as the winds shift more northwesterly and 
become more perpendicular to the long axis of the lake. Upslope 
into the terrain of far northwest Pennsylvania and far western NY 
for Lake Erie's bands and into the Tug Hill east of Lake Ontario 
will locally further enhance the lift and ability of the bands to 
produce incredibly heavy snowfall.

Storm total snowfall could approach 6 feet east of Lake Ontario and
2-4 feet southeast of Lake Erie. Off of the upper lakes, the
broader geography of the larger lakes makes single band formation
much more difficult, so multiple smaller bands of less heavy snow
are likely to impact portions of the U.P. and the northwestern
lower peninsula. Snowfall amounts over a foot are expected. PWPF
values show a 40-50% chance of 30 inches or more of snow in and
around Watertown through Sunday afternoon, and a 20-30% chance of
30 inches or more of snow for portions of northern Chautauqua
County, NY along I-90 southwest of Buffalo.

The latest Winter Storm Severity Index values show extreme impacts
are expected through Monday in the Watertown, NY area east of Lake
Ontario and along the I-90 corridor from the Buffalo southtowns
through Ashtabula, OH, including through Erie, PA southeast of Lake
Erie. Travel will remain extremely dangerous to near impossible
with numerous road closures. 

The ongoing lake-effect snow is a subject of the Key Message linked below.


...Missouri to West Virginia... 
Days 1-2...

A weak disturbance tracking with the jet around the broad trough in
the eastern U.S. will intensify as it approaches the base of the
trough. It will pick up some moisture as it moves along a strong
front. A very weak clipper low will develop, likely causing a
narrow area of snow in the form of a few bands from Missouri east
up the Ohio Valley and into the central Appalachians. Much of the
guidance suggests there will be two separate areas where the snow
is likely to be heaviest...one from Missouri across a portion of
southern Illinois and into southwest Indiana, and a second into
West Virginia. A local maximum of moisture from Missouri into
Indiana will combine with the weak surface development and upper
level jet streak and shortwave to produce the thin corridor of
potentially heavier snow for a period of a few hours. Meanwhile
into West Virginia upslope into the Appalachians will be the
primary forcing allowing the development of briefly heavy snow.

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

Wegman

...Winter Storm Key Messages are in effect. Please see current
Key Messages below...

https://www.wpc.ncep.noaa.gov/key_messages/La...

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