Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
337 PM EST Fri Dec 11 2020

Valid 00Z Sat Dec 12 2020 - 00Z Tue Dec 15 2020


Day 1...
...Central Plains to the Upper Great Lakes...
A deformation band associated with an upper level shortwave moving
through the central Plains will continue to develop through the
evening hours, with rain changing to snow from eastern Nebraska to
southern Wisconsin.  Low-to-mid level frontogenesis is expected to
support periods of banded precipitation, with several HREF members
indicating several hours of snowfall rates of 0.5-1 inch/hr during
the overnight hours across southwestern to northeastern Iowa and
southern Wisconsin.   Heavy, wet snowfall is likely on Saturday
across portions of central to northern Lower Michigan.  Guidance
shows the upper level shortwave assuming a negative tilt as it
moves from the mid Mississippi valley toward the Great Lakes
Saturday morning.  Coupled upper jet forcing centered over the
region, combined with low-to-mid level frontogenesis on the north
side of a deepening surface cyclone will support heavy snow, with
several HREF members indicating snowfall rates of over an inch/hr
persisting for several hours across northern Lower Michigan on
Saturday.  The latest WPC PWPF shows 50 percent or greater
probabilities for snow accumulations of 8-inches or more across
much of the region.

...Southern Cascades/Sierra to the Central Rockies...
An upper low moving into the Pacific Northwest will continue to
slide southeast -- moving across the northern Rockies overnight
and into the High Plains on Saturday.  Meanwhile, a well-defined
mid level shortwave/upper jet to its southwest will dig across the
Great Basin into the Four Corners region.  Moist, moist
northwesterly flow with an embedded weak wave will support
organized precipitation across the Sierra Friday night into early
Saturday; however, rising snow levels ahead of the wave will keep
heavy accumulations largely confined higher terrain.  Meanwhile,
left-exit region upper jet forcing and a strong low-to-mid level
front pushing southeast will support snow shifting rapidly from
Cascades through the Great Basin to the central Rockies.  While
this system will be fast moving, several inches of snow is
possible across portions of the higher terrain.  Areas impacted
are likely to include the central and northern Nevada mountains,
the Wasatch and the western Colorado ranges.

Day 2...
...Great Lakes and Northeast...
Snow is expected to diminish across northern Lower Michigan as the
low moves east of the region Saturday night.  Meanwhile, warm
advection precipitation east of the system will likely begin as a
wintry mix across portions of northern New York and New England. 
Models continue to show a weak wave developing along the Maine
coast, which will help to keep the low level cold air in place --
raising the potential for significant ice accumulations across
portions of northern New Hampshire and Maine, with several inches
of snow possible across far northern Maine.  WPC PWPF continues to
show 50 percent or greater probabilities for ice accumulations of
0.10 inch or more from northern Hampshire to north-central Maine.

...Central Rockies to the Ozark Region...
The previously noted shortwave digging into the Four Corners
region on Saturday is expected to swing east across the Rockies
into the High Plains Saturday night.  This will support snow
spreading east from the Colorado and northern New Mexico ranges
into the High Plains Saturday night into early Sunday.  A period
of strong upper forcing along with low-to-mid level frontogenesis
is expected to support a swath of at least a few inches from
southeastern Colorado through southeastern Kansas and the
Panhandle region toward central Oklahoma.  Models begin to differ
with the timing of the system as it moves east on Sunday, with the
faster non-ECMWF guidance bringing at least light accumulations as
far east as the Ozarks by late Sunday.

...Pacific Northwest/Northern California...
Mid level energy/an upper jet moving south of an upper low
centered over the Gulf of Alaska is expected to move into the
region by early Sunday.  This system is expected to produce
several more inches of snow across the higher elevations of the
Olympics and Cascades, with some potential for accumulating ice
across the lower elevations of interior western Washington and
northern Oregon.

Day 3...
...Pacific Northwest and Northern California to the Northern and
Central Rockies...
Mountain snows are expected to continue across the Pacific
Northwest and Northern California while spreading east into the
northern Rockies as an upper trough shifts east across the region
Sunday night into Monday.  Overall, not expecting widespread
additional heavy snowfall; however, some higher elevation areas
could see locally heavy accumulations.

...Central Appalachians...
An area of low pressure developing in response to the previously
noted shortwave moving into the Plains on Sunday is expected to
lift from the Southeast into the southern Mid-Atlantic Sunday
night into early Monday.  Given track and timing differences in
the models, forecast confidence in the details is low; however,
there are several guidance members showing enough cold air
remaining in place on the backside of the system to support at
least light snow accumulations, with some potential for several
inches, across the higher elevations of the central Appalachians.

Pereira