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

Valid 00Z Fri Dec 20 2024 - 00Z Mon Dec 23 2024

...Midwest to Great Lakes...
Day 1...

A progressive Alberta Clipper producing a swath of heavy snow over
Wisconsin today will track into the Ohio Valley tonight before
dissipating in the central Appalachians. Some of the clipper's
energy will transfer to a developing Nor'easter well offshore of
the eastern US. Modest 850mb WAA along the warm front and 850mb
theta-e wrapping around the northern flank of the storm is
prompting the area of heavy snow from Madison to just north of
Milwaukee. The low has already begun weakening, so the area of
heaviest snow has been shrinking.

Nevertheless, some snowfall could be lake-enhanced along the
shores of Wisconsin and Illinois as northerly winds advecting -10C
850mb temps race over Lake Michigan tonight. 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 through the Detroit metro
area. 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
Minnesota to a small swath of northern Michigan. There are some
embedded Moderate Impact areas, primarily in central Wisconsin.
Moderate Impacts imply these areas can expect hazardous driving
conditions with some potential closures and disruptions to
infrastructure. WPC probabilities sport moderate chances (40-70%)
for snowfall totals >4" from the Door Peninsula of Wisconsin and
northeastern portions of the L.P. of Michigan north of Saginaw.

...Central Appalachians to Northeast....
Days 1-2...

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 in Downeast Maine, but how close to the Maine
coastline the storm gets remains lower in confidence. WPC
probabilities suggest the eastern-most portion of Downeast Maine
has moderate chances (40-70%) for snowfall of 4 inches or more
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 (10-30%) 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 tonight 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 somewhere
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/Wegman