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

Valid 12Z Sun Dec 17 2023 - 12Z Wed Dec 20 2023

...Upper Midwest and Great Lakes to the Central and Southern
Appalachians...
Days 1-2...
Models continue to present a strong signal for widespread snow
showers and possible snow squalls impacting areas from the Great
Lakes through the Ohio Valley and into the central Appalachians
Monday and Monday night.  Locally heavy snow accumulations can be
expected, especially southeast of lakes Michigan and Erie and in
the upslope regions of the Allegheny Mountains. 

A powerful surface low moving from the eastern Gulf of Mexico to
the Southeast Coast this morning is forecast to track quickly
north along the East Coast, reaching the St Lawrence Valley by
Monday evening.  The heavy rain associated with this system is
forecast to remain centered east of the region.  However, a
shortwave trough that is currently centered over northern Alberta
and Saskatchewan is forecast to dive southeast, carving out a
sharp, negatively-tilted upper trough over the western Great Lakes
by early Monday before developing a closed-low farther east over
the Mid Atlantic and Northeast on Tuesday.  This will bring
sharply colder air across the still warm lakes.  Steepening lapse
rates and lift through the DGZ will support lake effect snow
showers with locally intense rates. 

While widespread heavy accumulations are not expected, some areas
are likely to see locally heavy amounts.  A period of deep
northwest flow will offer a multi-lake connection that will
support bands of heavier snow developing southeast of lakes
Michigan and Erie.   WPC PWPF shows moderate to high probabilities
for accumulations of 4 inches or more in these areas.

Heavy accumulations are also expected in the upslope regions of
the Allegheny Mountains from the southwestern Pennsylvania to
southern West Virginia.  WPC PWPF shows moderate to high
probabilities for accumulations of 4 inches or more extending
along this region, with an embedded area of moderate to high
probabilities for amounts of 8 inches or more centered along the
West Virginia mountains. 

The NAM and GFS continue to reflect the potential for convective
snow showers and isolated snow squalls developing in the wake of
the front -- shifting southeast from the Upper Midwest later today
and tonight to the upper Great Lakes, Ohio Valley, and the central
to southern Appalachians on Monday and Monday night.  These will
have the potential to produce brief, but intense snowfall and
windy conditions that will limit visibility and create hazardous
travel conditions.  Even apart from where any convective elements
may develop, a strong pressure gradient developing in the wake of
the low moving into eastern Canada and high pressure over the
Plains will support strengthening, gusty winds across the entire
region on Monday.

On Tuesday, snow is expected to diminish from west to east as high
pressure over the Plains shifts east and centers over the eastern
U.S. through the remainder of the period.

...Sierra...
Days 2-3...
An upper low currently centered over the eastern Pacific is
forecast to transition to an open wave, with the remnant energy
spreading inland ahead of a second low dropping southeast from the
Gulf of Alaska Monday night.  This second system is forecast to
remain offshore, but drop south along the Northern California
coast late Tuesday into early Wednesday.  While moderate to heavy
precipitation is expected to develop across much of Northern
California on Monday, snow levels beginning above 7000ft and
climbing to above 8000ft in many locations will limit the
potential for any widespread heavy snowfall amounts through early
Tuesday.  Then as the upper low drops south, snow levels dipping
back below 8000ft will broaden the heavy snowfall footprint on
Tuesday into early Wednesday.

Pereira