FOUS11 KWBC 291934
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
234 PM EST Fri Nov 29 2024

Valid 00Z Sat Nov 30 2024 - 00Z Tue Dec 03 2024

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

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

Mid-level low remains stalled over James Bay into Saturday before=20
drifting southeast to northern New England Sunday night and then
offshore into the Atlantic just beyond this forecast period
/Tuesday morning/. Repeated shortwave troughs and accompanying
vorticity lobes shedding around this low and over the Great Lakes=20
will provide periodic enhancement to the flow and accompanying CAA,
resulting in rounds of heavy lake effect snow (LES). Through D1 and
into D2, WNWly flow prevails across the Great Lakes driving=20
intense lake effect snow bands in the favored belts across the
U.P., NW lower peninsula, and then east of Lakes Erie and Ontario.
Extremely intense single bands east of the latter two lakes are
likely, aided by upstream moisture connection off of Lakes Superior
and Huron, with strong multi-bands expected elsewhere. As the flow
begins to shift to a more N/NW direction Sunday night and Monday,

This setup is primed for intense and widespread LES. Lake surface
temperatures of +8C to +15C (GLERL) are well above normal after a
warm fall, so with 850mb temps crashing to as low as -12C to -15C,
lake-850m temperature gradients will reach +20C or more. This will
drive lake-induced instability exceeding 1000 J/kg, and inversion
levels above 10,000 ft. Combine that with low-level convergence
along the lakes, and bands will easily produce 2-3"/hr rates,
especially downstream of Lakes Erie and Ontario where lighting and
thunder is also likely. Where these bands persists, snow will
accumulate exceptionally quickly, producing near impossible travel,
reflected by corridors of extreme impacts in the WSSI. The most
intense snowfall is likely D1 and D2, with heavy but less intense,
and more variable snow, dropping south with the wind shifts by D3.

WPC probabilities on D1 and D2 are high (>70%) for 12+ inches
downwind of Lakes Ontario and Erie, and high (>70%) for 8+ inches
along the other lake shores during the same time period. It is
likely that some places will receive several feet of snow during
this window, with the highest amounts of 3-4 feet expected near
Watertown, NY and the Tug Hill Plateau, and along portions of the
Chautauqua Ridge and surrounding neighborhoods. D3, the intensity
wanes slightly as the winds back more to the north, but heavy snow
is again likely with WPC probabilities for more than 8 inches
reaching as high as 50-70% east of Lake Erie, and 30-50% for 6+
inches elsewhere. Storm total snowfall of 5-6 feet is possible
during the next 3 days.

The arctic-sourced cold and LES off the Great Lakes through the=20
weekend is the subject of the Key Messages linked below.=20


...Missouri to West Virginia...=20
Days 1-2...

A positively tilted but potent shortwave will traverse through the
broad cyclonic flow encompassing the base of a longwave trough over
the eastern CONUS to produce a stripe of moderate to heavy snow
from Missouri to the Central Appalachians. This shortwave will
combine with modest upper diffluence along a residual baroclinic
boundary to drive weak cyclogenesis as a clipper-type low crosses
the region. This low is likely to move from Missouri Saturday aftn
to WV by Sunday morning, with accompanying downstream moisture
advection and isentropic ascent producing a swath of moderate
snowfall. The presence of the upper jet streak responsible for the
upper diffluence and warm/isentropic ascent will drive some
enhanced fgen which could produce briefly heavier snow rates at
times as ascent favorably intersects the DGZ. However, overall
modest available moisture and the progressive nature of the forcing
should limit accumulations in most areas.=20

The heaviest snowfall is likely in the vicinity of St. Louis, MO=20
where conditional instability and greater DGZ depth overlap, with=20
a secondary max expected in the higher terrain of WV due to higher=20
SLRs and favorable upslope flow persisting after the low passes.=20
With cold air preceding this system, any snow should efficiently=20
accumulate leading to periods of hazardous travel during the=20
heaviest snow. WPC probabilities are 10-30% for 4+ inches on D1=20
near St. Louis, and peak above 50% in the WV Appalachians on D2.

The probability of significant ice across the CONUS is less than
10 percent.=20


Weiss



...Winter Storm Key Messages are in effect. Please see current=20
 Key Messages below...

https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late=
stKeyMessage_1.png__;!!DZ3fjg!6sq7zvzGkogktUKTk1Ckd16KekKCyx8eHDqZdXesWSlxB=
70nYz5EfBVzVt3lBNEoA41p40dEsC6YjnXbDttWCak5A8w$=20

$$

=3D =3D =3D
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