Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
443 PM EST Sat Dec 12 2020

Valid 00Z Sun Dec 13 2020 - 00Z Wed Dec 16 2020


Day 1...
...Pacific Northwest/Northern California...
After a brief interlude, unsettled weather is expected to return
to the region by early Sunday as shortwave ridging gives way to
approaching mid level energy and an upper level jet.  This will
bring another round of mountain snows to the Olympics, Cascades
and Sierra, with the potential for heavy accumulations across
higher terrain.  Models do continue to present some signal for
freezing rain and accumulating ice across portions of the Columbia
River Gorge, as shallow easterly flow helps to maintain low level
cold air.

...Southern and Central Rockies into the Plains...
Snow are expected to spread east across portions of the central
and southern Rockies as a fast moving upper trough continues to
amplify and dig southeast through the Great Basin into the region
overnight.  Given the progressive nature of the system, widespread
heavy totals are not expected; however, portions of the southern
Colorado and northern New Mexico ranges, including the Sangre de
Cristo Mountains, could see several inches.   Then beginning
Saturday night and continuing into Sunday, models continue to
signal a heavy snow event unfolding farther east over the Plains. 
In addition to strong forcing aloft, low-to-mid level
frontogenesis is expected to support a swath of moderate to heavy
snow developing from southeastern Colorado into southwestern
Kansas and the northern Panhandle Region late Saturday into early
Sunday, before shifting farther east later in the day.  Several
HREF members indicate banded snowfall, with 0.5-1 inch/hr rates,
developing over southeastern Colorado, southeastern Kansas and the
Oklahoma Panhandle overnight, the drifting southeastward into
central Oklahoma during the day. 

...Great Lakes/Northeast...
Ongoing heavy snow across northern Lower Michigan is expected to
wane overnight as the associated area of low pressure moves
northeast of the region.  Meanwhile, warm advection precipitation
east of the system will continue to move across the Northeast. 
With the exception of northern Maine, this will be mainly a rain
event for the region.  Easterly winds between a weak low
developing along the coast and high pressure to the north will be
enough to maintain the low level cold air -- supporting a wintry
mix with accumulating ice likely from northwestern into central
Maine, and a few inches of snow farther to the north across far
northern Maine.

Day 2...
...Pacific Northwest and California to the Northern and Central
Rockies...
Mountain snows are expected to continue early in the period across
the Northwest and Northern California, while spreading east into
the northern Rockies as an upper trough continues to move across
the region Sunday night into early Monday.  Models show the system
continuing to move progressively east, bringing snows across the
Great Basin and into the central Rockies on Monday.  Meanwhile,
drying conditions are expected to the west across the Northwest
and Northern California as another shortwave ridge moves across
the region late Monday.  There is the potential for additional
locally heavy accumulations from the Sierra and Cascades to the
Wasatch.

...Central Appalachians into the Northern Mid Atlantic and
Northeast...
As an upper level shortwave continues to move across the southern
Plains, low pressure is expected to develop over the lower
Mississippi valley and begin to track northeastward Sunday night. 
As the upper level wave continues to race eastward and assume a
negative tilt, models show the surface low continuing to deepen as
it the approaches the southern Mid Atlantic coast on Monday. 
Cooling on the backside of the system is expected to be sufficient
for rain changing to snow, with several inches of accumulation
possible across portions of the central Appalachians on Monday. 
There is also some potential for at least light accumulations
extending farther northeast across portions of the northern Mid
Atlantic into the Northeast as the system moves offshore on
Monday.  However, model spread with respect to the strength and
track of the system and the resulting northern extent of the
precipitation shield, have resulted in limited confidence in the
forecast details and potential impacts.

Day 3...
...Central Plains...
An upper trough moving across the West on Monday is expected to
move into the Plains on Tuesday.  This will likely bring another
round of accumulating snows to the region with snows developing
initially along an elongated trough extending from western
Oklahoma into central Nebraska on Tuesday before focusing farther
to the south across central into eastern Oklahoma late in the day.

Pereira