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/LatestKeyMessage_1.png

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FOUS11 KWBC 020859
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
359 AM EST Mon Dec 2 2024

Valid 12Z Mon Dec 02 2024 - 12Z Thu Dec 05 2024

..Great Lakes... Day 1...

*** Heavy Lake-Effect Snow continues through tonight in Michigan,
through Tuesday for eastern Great Lakes ***

Reinforcing shortwave trough axis is over central MN early this
morning and will continue to round the longwave trough axis over
the interior Northeast by crossing the Midwest today and track near
the VA/NC line tonight before moving offshore.

Michigan Lake Effect...
Flow over Lakes Superior and Michigan will remain veer more NNWly
today behind the shortwave trough axis and maintain heavy snow over
the eastern U.P. and along the western shore of the L.P. into far
northern IN. Day 1 PWPF for >6" is 40-70% around Grand Traverse Bay
and far SW MI to the IN border.

Eastern Lake Effect...
Flow over the eastern Great Lakes continues to veer NWly with the
trough passage this morning. This means that while the single band
that had been pushing into the eastern shore of Lake Erie on Wly
flow will be disrupted, the NWly flow will continue to produce
heavy LES over the Chautauqua Ridge from fetch over Lake Huron and
then over Lake Erie. Day 1 PWPF is 60-90% over that portion of
extreme western NY with probabilities dropping off toward Erie PA.
Banding off Lake Ontario is expected to continue for the Syracuse
area today with an additional 6" possible.

Day 2...
High pressure currently centered near NW ND will shift down the
Mid- Mississippi Valley Tuesday with a more amplified ridge
extending north over the Great Lakes. Behind this ridge axis, flow
over Lakes Michigan and Superior quickly shifts to the SW which
ends the LES bands from overnight. However, as this new fetch
saturates, warm air advection induced snow develops across the U.P.
tonight, particularly as a surface low ahead of the next wave
approaches Lake Superior from the NW late tonight and pivots east
Wednesday. The air remains cold enough for SWly flow driven lake
enhanced snow into the far eastern U.P. Day 2 PWPF for >6" is
limited to the Keweenaw Peninsula and near the Mackinac Strait.

The ridge axis does not shift over the eastern Great Lakes until
Tuesday night, so LES will continue and essentially shift back
toward a single-band appearance on SWly flow, though the warm air
advection makes for a wetter snow than recent days. Day 2 PWPF is
40-60% around the Chautauqua Ridge with low probs for >4" for the
south towns of Buffalo.

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

Day 3...
The low over Ontario further develops and sends a strong cold front
over the Great Lakes Wednesday into Thursday. 850mb temps quickly
drop into the DGZ behind the front, so LES resumes with a vengeance
on NWly flow through the day over Michigan and that night/Thursday
morning for the eastern Great Lakes. Day 3 PWPF >6" is 30-70% for
NWly snow belts across the northern U.P. and the northern L.P.
Marginal thermals for snow and warm air advection limit eastern
Great Lakes snow Wednesday, but quickly shifts snowy overnight. Day
3 PWPF for >6" is around 20% off Lake Erie and prolonged moderate
snow over the Tug Hill makes for 40-60% probs there.

LES looks to continue over the eastern Great Lakes on the NWly flow
into Friday.

...New England... Day 3...

The upper low tracks across northern Lake Huron Wednesday night
with strong warm air advection and increasing precip into an
initially cold airmass over New England allowing moderate to
locally heavy snow over the terrain of the Greens/Whites/and much
of Maine. Day 3 PWPF for >6" 30-60% for these areas. The low
crosses Maine on Thursday when there is a greater snow threat for
interior New England.

...Southern Appalachians... Day 1...

A clipper-type trough currently over central MN shifts to eastern
TN by this evening. Upslope flow enhances precip which is cold
enough for snow and SLRs around 17:1 to produce 30-60% probs for

4" snow tonight for mainly the western slopes of the Great Smokey Mtns.

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

Jackson

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

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png

$$
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