FOUS11 KWBC 021945
QPFHSD

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

Valid 00Z Tue Dec 03 2024 - 00Z Fri Dec 06 2024

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

Reinforcing shortwave trough axis over the Ohio Valley this
afternoon will continue to round the longwave trough axis over the
Northeast by tracking near the VA/NC line tonight before moving
offshore. Flow will continue to be NNWly over the Lakes tonight
before shifting more westerly on Tuesday ahead of the next
approaching shortwave.

Michigan Lake Effect...
NNWly 850mb flow up to 20 kts over Lakes Superior and Michigan
will continue to support locally heavy snow over the eastern U.P.
and along the western shore of the L.P. into far northern IN. Day 1
PWPF for >4" is 40-70% around Grand Traverse Bay and far SW MI to
the IN border.

Eastern Lake Effect...
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 for at least 6" is 60-80% over that portion of extreme western
NY with probabilities dropping off toward Erie PA. Banding off
Lake Ontario is also expected to continue for the Syracuse area
into tonight with an additional 4" possible.

Day 2...
High pressure currently centered over 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. Tuesday night,
particularly as a surface low ahead of the next wave approaches
Lake Superior from the NW late early D2 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 high (>70%)
for the far eastern U.P. and medium (40-60%) for the Keweenaw
Peninsula.


The ridge axis does not shift over the eastern Great Lakes until
early Wednesday morning, 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 for >4" is 20-50% around the Chautauqua Ridge with low
probs for the south towns of Buffalo. Higher probs (50-70%) are
found near the Tug Hill Plateau.


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. Snow-liquid ratios would
normally rise to around 20:1 or higher, but very strong 850mb winds
of 40-55 kts could fracture dendrites and lead to areas of blowing
snow. Day 2-3 PWPF >6" is >80% 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 60-80%
off Lake Erie and prolonged moderate snow over the Tug Hill makes
for >90% probs there.

Additionally, snow squalls are likely along and just behind a
strong cold front crossing the Upper Great Lakes on Wednesday and
into the L.P. of Michigan by Wednesday night as 3-6 km AGL lapse
rates rise to around 7 C/km underneath the arctic airmass.This
could lead to intense multi- band convective squalls enhanced by
flow off Lake Superior and Lake Michigan Wednesday evening.
Potential also exists for snow squalls across the northern Mid-
Atlantic and Northeast by Thursday, but timing and mode is more
uncertain. Either way, gusty winds with maximum values exceeding 40
mph are likely to be widespread and persistent between the
northern Plains and central Apps/Northeast, which could lead to
lowering visibility where snow is falling and where fresh snow
cover exists.

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. The low crosses ME on Thursday when there is a greater
snow threat for interior New England. Day 3 PWPF for >6" 30-60% for
the higher terrain of VT/NH, with a corridor of 60-80% extending
from the central highlands to northern portions of ME. The low
crosses ME on Thursday when there is a greater snow threat for
interior New England.


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

A clipper-type trough currently over the Ohio Valley shifts to
eastern TN by this evening. Upslope flow enhances precip which is
cold enough for snow and SLRs around 17-18:1 to produce 30-60%
probs for >4" snow tonight for mainly the western slopes of the
Great Smokey Mtns. Enough lift directly beneath the shortwave
could also lead to minor snowfall accumulations (<1") Tuesday
morning across central NC/SC.


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


Snell/Jackson






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