Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
331 PM EST Thu Jan 07 2021

Valid 00Z Fri Jan 08 2021 - 00Z Mon Jan 11 2021


Day 1...
...Tennessee Valley to the Southern Appalachians...
A closed low currently moving east across the lower Mississippi
valley is expected to continue into the Tennessee valley
overnight.  Rain changing to snow within the associated
deformation band is expected to result in light snow accumulations
across portions of Middle Tennessee (less than an inch).  Some
relatively higher accumulations are still expected across portions
of the Cumberland Plateau.  Latest WPC PWPF shows high
probabilities for accumulations of 1-2 inches across portions of
the plateau, with some potential for locally heavier
accumulations.  The heaviest event totals are still expected to
center along the southern Blue Ridge, especially along the eastern
slopes.  As the low-to-mid level center moves across the
Southeast, easterly flow into the region is expected to increase
-- which along with favorable upper forcing is expected to help
support heavy snowfall beginning Thursday night.  With several 12Z
guidance members shifting their axis of heavier QPF farther south,
probabilities for heavy snowfall accumulations have decreased
across the southwestern Virginia and northeastern North Carolina
mountains.  Locally heavy accumulations still appear likely
however farther south over portions of southwestern North Carolina
to the Tennessee border.  There is still some question with how
far significant accumulations may extend east into the Piedmont. 
Some guidance members, (the NAM especially), show low-to-mid level
frontogenesis supporting heavier precipitation rates out along the
North Carolina/Virginia border.  While the NAM especially may be
overdone, do believe there is good potential for snowfall
accumulations on the order of an inch or two extending out into
north-central North Carolina to south-central Virginia Friday
morning.

...Pacific Northwest...
A well-defined shortwave/upper jet positioned along the base of a
broader scale trough are expected to move into the Pacific
Northwest Friday morning.  A period of deep westerly flow, along
with favorable upper jet forcing, is expected to support organized
precipitation, including mountain snows along the Cascades into
the Shasta -Siskiyou region, with the heaviest accumulations most
likely along the higher elevations of the central to southern
Oregon Cascades. 

Day 3...
...Southern Rockies and Southern Plains...
The previously noted shortwave entering the Northwest on Day 1 is
forecast to dig southeast across the Great Basin and into the Four
Corners region on Saturday.  As the system moves east into the
southern Rockies, increasing low level southeasterly flow, along
with left-exit region jet forcing will support precipitation
developing by Saturday evening from the Panhandle region into the
southern Rockies.  Precipitation is forecast to continue through
the overnight across portions of the Panhandle region and eastern
New Mexico, with cold air aloft sufficient for snow across the
region.   As the system moves farther east, models show a closed
low developing over western Texas, with precipitation shifting
east across the state on Sunday.  Dynamic cooling is expected to
help draw the rain-snow line farther southeast on Sunday,
introducing the potential for significant snowfall accumulations
into portions of Northwest Texas by Sunday evening.  

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Pereira