Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
431 PM EST Wed Dec 30 2020

Valid 00Z Thu Dec 31 2020 - 00Z Sun Jan 03 2021

Day 1...
...Pacific Northwest to the Northern Rockies...
As a deep, compact low moves toward the northern tip of Vancouver
Island, a well-defined mid-level shortwave/upper jet moving to its
south is expected to move inland near the California-Oregon border
this evening.  This will help support the development of organized
heavier precipitation farther south across Oregon into
northwestern California.  As this shortwave drops quickly to the
southeast overnight, the previously noted low is expected to
quickly weaken and drop southeast across the Pacific Northwest on
Thursday -- supporting additional heavy snows from portions of the
northern Cascades to the northern Rockies.  WPC PWPF shows high
probabilities for additional snow accumulations of 8-inches or
more for portions of the Washington and Oregon Cascades, the Blue
and Wallowa mountain region in southeastern
Washington/northeastern Oregon, and the Idaho Panhandle ranges.

...Southern Plains...
An upper low will continue to deepen as it moves across northern
Mexico Wednesday night into Thursday morning, assuming a negative
tilt as it lifts into South Texas on Thursday.  The depth of the
cold air is expected to remain sufficient for snow on the
northwest side of the developing comma head -- with several inches
of accumulation likely across portions of West Texas.  In addition
to strong upper forcing, low-to-mid level frontogenesis is
expected to enhance the potential for heavy snowfall rates. 
Several members of the 12Z HREF show the threat for snowfall bands
producing over an inch/hr drifting slowly across the Davis
Mountains to the Stockton Plateau region.  WPC PWPF shows very
high probabilities for accumulations of 8-inches or more across
portions of this region, with lower probabilities extending
farther east into the western Edwards Plateau and Concho Valley.
An icy mix is more likely farther to the east, with accumulating
ice expected from the Texas Hill Country northward to eastern
Oklahoma and the Ozark region.

Day 2...
...Southern and Central Plains to the Mid Mississippi Valley and
Southern Great Lakes...
A deep low centered over North Texas Thursday evening is forecast
to lift north-northeast across the Ozark region and into the mid
Mississippi valley on Friday -- drawing plenty of moisture and
warm air aloft into the eastern Plains, mid Mississippi valley and
Great Lakes.  Strong upper forcing along with low-to-mid level
frontogenesis is expected to produce widespread precipitation
moving north across Oklahoma, eastern Kansas and the lower
Missouri and mid Mississippi valleys Thursday night into Friday
morning, with shallow cold air supporting a wintry mix and
accumulating ice.  While differences in model thermal profiles and
run-to-run differences in the track of the system have limited
forecast confidence in the details, there is still a fair amount
of certainty that by Friday morning, significant ice accumulations
can be expected across portions of eastern Kansas and across
northern and central Missouri, including the KC metro.   WPC PWPF
continues to show high probabilities for ice accumulations of
0.10-inch or more, with some areas within this area likely to
receive over a 0.25-inch.    As the northern edge the
precipitation shield continues to move north on Friday, the
potential for accumulating is likely to decrease some with the
introduction of daytime heating.  However, it is expected that the
cold air will be sufficient for a wintry mix, including some
freezing rain, lifting north across central to northern Illinois,
northern Indiana and southern Lower Michigan during the day on
Friday.  Meanwhile, deepening cold air on the backside of the
system is expected to support a changeover to snow before
precipitation ends.  While widespread heavy snow accumulations are
not expected, at least an inch or two can be expected from the
eastern Texas Panhandle and Northwest Texas to northwest Missouri
and southern Iowa.

...Central Appalachians...
As the area of low pressure approaching the Great Lakes continues
to direct moisture and warm air aloft from the central into the
eastern U.S., models show high pressure over the central U.S.,
shifting into the Northeast, with a cold air damming signature
setting up along and east of the central Appalachians Friday
morning.  This is likely to produce wintry precipitation, with
areas of accumulating ice.  Guidance shows the biggest threat for
significant ice accumulations centered from western Maryland into
central Pennsylvania.

Day 3...
...Pacific Northwest...
Another upper level shortwave and its associated frontal band are
expected to bring another round of wet weather, including heavy
mountain snow, into the region Friday night.  While this system is
expected to weaken and move progressively east on Saturday.  Deep
southwesterly flow ahead of the next approaching system will
continue to channel deeper moisture and the potential for heavy
precipitation into northwestern Washington through late Saturday. 
Heavy snowfall totals on the order of a foot or more can be
expected across the higher elevations of the Olympics and northern
Cascades.

...Northern Mid-Atlantic/Northeast...
As low pressure continues to move northeast across the Great
Lakes, a wintry mix will continue to spread north from the central
Appalachians and northern Mid-Atlantic to the Northeast late
Friday into Saturday.  A retreating cold-air wedge is expected to
support freezing rain at the onset, with accumulating ice expected
from central Pennsylvania northeastward to western New England. 
High probabilities for 0.10-inch of ice or more are shown from
Catskills to the Adirondacks, Berkshires and southern Greens.  A
secondary low developing along the northern New England coast on
Saturday, will help maintain the cold air across interior Maine,
where snow is expected to remain the predominate precipitation
type through the event.  WPC PWPF shows high probabilities for
snow accumulations of 4-inches or more across much of central and
northern Maine.

Pereira