Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
413 PM EST Wed Jan 06 2021

Valid 00Z Thu Jan 07 2021 - 00Z Sun Jan 10 2021


...Ozark Region to the Southern Appalachians...
Days 1-2...
A well-defined southern stream shortwave will continue to swing
east across the northern Texas, with most models showing a closed
mid-to-upper level center developing near the ArkLaTex region this
evening.  Rain changing to snow, with light accumulations, is
expected across the Ozarks, as the system moves to the south
across Louisiana overnight. 

As the system moves east, mostly rain is expected during the day
across the Tennessee valley on Friday.  A changeover to snow is
expected to result in mainly light accumulations across Middle
Tennessee into Kentucky during the evening hours.  But then by
late Thursday evening and continuing through the overnight hours
into Friday, the potential for significant snow is expected to
increase across portions of the Cumberland Plateau and the
southern Appalachians.  The biggest threat for heavy amounts is
expected to center across the southern Blue Ridge, with lighter
totals across the Cumberland Plateau and farther east into the
western Piedmont.  As the low-to-mid level center moves across
Georgia and South Carolina, easterly flow north of the low-to-mid
level center is expected to support moderate to heavy snowfall
across the upslope regions.  The latest PWPF indicates the
widespread potential for snow accumulations of 4-inches or greater
across the southern Blue Ridge, with locally heavier amounts
expected.  Forecast confidence has been limited in part by
significant model spread with respect to QPF across the region. 
Overall, WPC preferred a wetter solution, with higher snow totals
than the GFS, across Tennessee into the Appalachians.  While the
NAM was more preferred in these areas, it is a significant outlier
east of the mountains, with heavier snow totals spreading out
across North Carolina and southern Virginia.  Given its current
outlier position, this solution was given little weight.

...Pacific Northwest and Northern California...
Day 2...
A well-defined shortwave/upper jet moving south of a closed low
approaching Vancouver Island is expected to support another round
of organized precipitation, including locally heavy mountain snows
across the southern Cascades and the Klamath Mountains on Friday. 
While widespread heavy totals are not expected, there is the
potential for accumulations of 8-inches or more across portions of
the higher terrain.

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

Pereira