Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
426 PM EST Sun Dec 13 2020

Valid 00Z Mon Dec 14 2020 - 00Z Thu Dec 17 2020


Day 1...
...Pacific Northwest/California to the Northern and Central
Rockies...
A deepening, fast-moving, upper trough is expected to bring
widespread snows, with the potential for locally heavy amounts
across the higher terrain as it moves from the West Coast to the
Rockies this period.  Greatest potential for heavy amounts is
expected to center along the Sierra, where left-exit region upper
jet forcing is expected to accentuate lift Sunday evening into the
overnight.  WPC PWPF shows high probabilities for additional
accumulations of 6-inches or more after 00Z, mainly for areas
above 5000 ft.  Generally lighter amounts are expected elsewhere;
however, there is the potential for locally heavy amounts across
the higher terrain.  Those areas include the northern Nevada and
southern Idaho mountains, as well as the Wasatch and Uintas.

...Central Appalachians into the Northern Mid Atlantic...
As a well-defined upper trough moves east across the lower
Mississippi into the Tennessee valley surface low pressure is
expected to deepen and track east of the southern Appalachians on
Monday.  Strong upper jet forcing, along with low-to-mid level
frontogenesis is expected to support widespread moderate to heavy
precipitation west of the low, with the strong dynamics and low
level northerly flow supporting rain changing to snow across the
portions of the central Appalachians, with several inches possible
across the higher terrain of eastern West Virginia.  While
widespread heavy amounts are not expected elsewhere, there is good
potential for light amounts, generally on the order of an inch or
two of accumulating snow, from the eastern West Virginia Panhandle
to southern New England.

Day 2...
...Pacific Northwest...
An weakening upper low is expected approach northern British
Columbia this period, will mid-level energy/an upper jet breaking
down a shortwave ridge to the south over the Pacific Northwest. 
Increasing onshore flow along with favorable forcing aloft will
support the return of organized precipitation across western
Washington, with heavy snow accumulations possible across the
higher elevations of the Olympics and northern Cascades.

...Central to Southern Plains...
Upper trough traversing the West on Day 1 will continue to move
progressively east - moving east of the Rockies early Tuesday. 
Models continue to show a swath of generally light snows
developing along an north-south, elongated trough/frontogenetical
axis extending from western Kansas into central Nebraska Tuesday
morning.  By late Tuesday consensus of the models shows steadier
precipitation reorienting farther to the south - closer to the
center of a mid-to-upper level low moving along the
Kansas-Oklahoma border.  Overall, expect accumulations to be
light, with WPC PWPF showing little potential for widespread
amounts of 4-inches or more.

Day 3...
...Southern and Central Appalachians/Mid-Atlantic...
A major winter storm is expected to develop on Wednesday,
impacting portions of region, with significant ice accumulations
and heavy snow. Precipitation will begin to develop across the
region by early Wednesday and become widespread by late in the day
as the previously noted trough over the Plains continues its
progressive track to the east.  Models are showing a classic cold
air damming signature, with a cold surface ridge extending from
eastern Canada through the Mid Atlantic east of the mountains.  A
developing coastal low will help to maintain the low level cold
air as it tracks from the Carolinas to southeastern Virginia. 
While guidance differs on the details, there is overall agreement
on the potential for freezing rain, with significant ice
accumulations, developing across portions of the Mid Atlantic on
Wednesday.  Latest WPC PWPF shows high probabilities for ice
accumulations of 0.10 inch or more occurring from the southern
Blue Ridge eastward into the North Carolina and southern Virginia
Piedmont.  Heavy snow is more likely to develop farther to the
north, with several inches of accumulation likely by late
Wednesday from northwestern Virginia and the eastern West Virginia
Panhandle into south-central Pennsylvania.  While the heaviest
amounts through late Wednesday will likely be well north and west
of the cities, accumulating snows are likely along the I-95
corridor from Washington to New York.

Pereira