FOUS11 KWBC 190856
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
356 AM EST Thu Dec 19 2024

Valid 12Z Thu Dec 19 2024 - 12Z Sun Dec 22 2024

...Midwest to Great Lakes...
Days 1-2...

A progressive Alberta Clipper producing a swath of heavy snow over
North Dakota tonight will track across the Upper Midwest today.
Modest 850mb WAA along the warm front and 850mb theta-e wrapping
around the northern flank of the storm will prompt a snow to come
down at 1-2"/hr rates in some cases from eastern ND to central
Minnesota and central Wisconsin. The higher end of those rates will
be harder to come by east of the Mississippi River as 850mb
frontolysis sets in, leading to a reduction in snowfall rates.
Still, the 500mb shortwave trough responsible for upper level
ascent will still preside over the region and allow for periods of
moderate-to-heavy snow through Thursday evening. In fact, some
snowfall could be lake-enhanced along the shores of Wisconsin and
Illinois as northerly winds advecting -10C 850mb temps race over
Lake Michigan. By Thursday night, the 850mb low will track through
northern Indiana with the best accumulating snow potential to the
north of the 850mb low track. This favors central and northern
Michigan Friday morning with a chance for the Detroit metro area to
see light accumulations Friday afternoon. The storm system will
gradually weaken into an open wave Friday afternoon as it
approaches the central Appalachians, effectively ending the period
of snowfall related to this Clipper in the Great Lakes by Friday evening.

The WSSI shows a large swath of Minor Impacts that stretch from
North Dakota and far northern South Dakota to as farther east as
northern Michigan. There are some embedded Moderate Impact areas,
primarily in parts of central North Dakota, around the Twin Cities
metro, and in eastern Wisconsin. Moderate Impacts imply these
areas can expect hazardous driving conditions with some potential
closures and disruptions to infrastructure. WPC probabilities sport
high chances (>70%) for snowfall totals >4" from central MN to the
Door Peninsula of Wisconsin. It is worth noting that there are
some localized low chance probabilities (10-30%) for snowfall
totals >8" in central MN and central WI, as well as around the
Green Bay area where lake-enhanced snowfall could result in
localized amounts approaching 10 inches.

...Central Appalachians to Northeast....
Days 2-3...

As the Alberta Clipper tracks through the Lower Great Lakes Friday
morning, a secondary 500mb vort max that raced south from south-
central Canada will begin to phase with the shortwave associated
with the Clipper. This process, starting as early Thursday night,
will lead to an amplification of the 500mb trough as it tracks
towards the Central Appalachians Friday morning. Meanwhile, off the
East Coast, increasing upper level divergence along a frontal
boundary will allow for low pressure to quickly organize and
strengthen Friday afternoon. As WAA increases along the front,
moisture will wrap around the low and lead to broad precipitation
shield just off the Northeast coast Friday night. This could result
in heavy snow along Downeast Maine, but how close to the Maine
coastline the storm gets remain lower in confidence. WPC
probabilities suggest the eastern-most portion of Downeast Maine
has low-to-moderate chances (30-50%) for snowfall >6" of snow
through Saturday evening. In addition, high pressure over Quebec
will work in tandem with the deepening storm system to tighten the
pressure gradient and strengthening low-level northerly winds off
the MA Capes. There is the potential for ocean-effect snow over
southeast MA Friday evening. WPC probabilities show low chances
(10-3) for snowfall >4", which does include the Boston metro area.

Throughout the rest of the Northeast, western NY and northern PA
will see the clipper's remnant 850mb circulation track overhead
with modest 850-700mb moisture and weak 850mb WAA will supply the
ingredients necessary for periods of snow late Thursday night and
into Friday. By Friday afternoon, the forecast over parts of the
northern Mid-Atlantic will hinge upon the strength of low-level
easterly flow and a convergence axis over the region, while
sufficient upper level divergence from the left-exit region of a
500mb jet streak arrives. Should these factors mesh together in
just the right way, a pivoting band of snow may organize some where
between northeast MD or the northern DelMarVa Peninsula on
northeast through eastern PA, the Poconos, and into the Tri-State
area. The latter is most favored for accumulating snowfall as WPC
probabilities show moderate chances (40-60%) for snowfall totals
surpassing 2" between Friday afternoon and early Saturday morning.

If there is one mountain range that is favored to witness heavier
snowfall, it is the central Appalachians in eastern WV and western
MD. As the remnant surface low of the Clipper heads east Friday
evening, its moisture source will track into the mountains at the
same time as northwesterly winds begin to increase. The end result
is upslope flow that causes periods of snow as far north as PA's
Laurel Highlands. WPC probabilities favor those windward slopes of
eastern WV with moderate chances (40-60%) for snowfall >4",
especially at elevations above 2000ft between 18Z Fri - 18Z Sat.
For all these areas mentioned in this section, the WSSI shows Minor
Impacts for impacted areas that would primarily lead to locally
hazardous travel conditions through Saturday morning.


The probabilities for significant ice accumulations are less than
10 percent.

Mullinax
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