Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
217 PM EST Sun Dec 17 2023

Valid 00Z Mon Dec 18 2023 - 00Z Thu Dec 21 2023

...Great Lakes, Ohio Valley, Central Appalachians...
Days 1-2...

A potent shortwave and accompanying vorticity lobe will streak
southeast out of Manitoba tonight and then amplify into a closed
low across the Mid-Atlantic Monday night and then shear off to the
north and eject the New England coast by Wednesday morning. This
will drive a surface cold front rapidly eastward on Monday, with
impressive CAA in its wake driving 850mb temperatures to as low as
-20C across the Great Lakes and -10C as far south as the central
Appalachians. This evolution will result in three areas of heavy
snowfall.

Across the Great Lakes, the strong CAA will move across still warm
lake temperatures to produce enhanced lake instability supporting
lake effect snow. This will occur first to the south of Lake
Superior along the U.P. of MI tonight, and then extend into the
western L.P. before stretching to south of Lake Erie on D2. The
forcing is impressive over the lakes and there will be the
additional enhancement of synoptic moisture beneath the upper
trough. At the same time, the CAA will drive raising inversion
heights and a deepening DGZ to support snowfall rates that could
reach 1-2"/hr at times, especially in the most focused bands, and
aided by upstream connections as flow moves across multiple lakes.
WPC probabilities for more than 4 inches day 1 are 50-80% across
the U.P., southwest L.P., and near Traverse City, MI, then
continuing across lower Michigan D2 before reaching 50-80% south
of Lake Erie and along the Chautauqua Ridge. By D3 the LES winds
down, but 2-day snowfall could reach 10 inches in a few locations.

This increasing cyclonic flow and CAA will also ramp up the
upslope snow potential across the central Appalachians from the
Laurel Highlands southward into NC. The greatest risk for heavy
snow appears to focus across WV where regional soundings are quite
impressive for intense snowfall and forcing sufficient to wring
out all available moisture. Snowfall rates within the upslope
could also reach 1-2"/hr, and this could become a very impressive
snowfall event for parts of WV before precipitation winds down by
D3. WPC probabilities for more than 4 inches exceed 90% in the
higher terrain of WV on D2, and local amounts above 12" are likely.

Lastly, there is increasing potential for widespread convective
snow showers and isolated snow squalls beneath the primary PV
anomaly and behind the front from Wisconsin, through the OH VLY,
and into the interior Mid-Atlantic on Monday. The snow squall
parameter gets quite high across this area, and soundings suggest
elevated instability and sufficient PBL winds to produce
convective snow showers as the column cools. A relative lack of
0-2km fgen may limit the potential for widespread snow squalls,
but at least isolated snow squalls are likely. Regardless, fast
moving snow showers with snowfall rates exceeding 1"/hr will
likely produce reduced visibility and snow covered roads at times.
The flash freeze potential appears modest due to surface
temperatures around 0C, but impacts are possible where any of
these stronger snow showers or squalls occur.


...Sierra...
Days 1-3...

Broad troughing across the Pacific will slowly amplify in response
to a shortwave dropping southward well off the west coast, with
this shortwave deepening into an amplified closed low west of the
Bay Area of California by 00Z/Thursday. Pinched flow south of this
feature will direct waves of enhanced IWV eastward and onshore CA,
with downstream divergence, subtle height falls, lobes of
vorticity, and, eventually by D3, upper diffluence in the LFQ of a
jet streak driving ascent across the area. IVT is progged by both
the GFS and ECENS to have a low probability of exceeding 500
kg/ms, highest D2, which will result in waves of precipitation
repeatedly spreading onshore through the forecast period. In
general, this precipitation will be accompanied by WAA within the
onshore flow, but a weak cold front may move across the area late
D2 into D3, bringing at least a subtle reduction in snow levels to
around 7000 ft. Otherwise snow levels are forecast to be 8000-9000
ft, indicating the most significant snowfall impacts will be
confined to the highest terrain of the Sierra, but some snowfall
impacts are likely at the higher passes including Tioga, Sonora,
and Carson. By D3 some light snow may occur across Donner Pass as
well. WPC probabilities across the Sierra peak above 50% for 6+
inches on D2 and D3, and 3-day total snowfall may exceed 2 feet in
the highest terrain.

Weiss