Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
352 PM EST Thu Dec 31 2020

Valid 00Z Fri Jan 01 2021 - 00Z Mon Jan 04 2021

Day 1...
...Southern and Central Plains to the Mid Mississippi Valley and
Southern Great Lakes...
A deep low currently moving into southern Texas 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
the Great Lakes.  Strong upper forcing along with low-to-mid level
frontogenesis is expected to produce widespread precipitation
moving north across Texas, 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 ahead of the approaching low.   The 12Z model
consensus showed an additional southern shift in the axis of
heavier ice accumulations, but overall significant ice
accumulations still appear likely from eastern Kansas, and across
much of central and northern Missouri, into central Illinois.   
WPC PWPF continues to show high probabilities for ice
accumulations of 0.10-inch or more across this region, with some
areas within this area likely to receive over a 0.25-inch.  
Meanwhile, deepening cold air on the backside of the system is
expected to support a changeover to snow before precipitation
ends, with low-to-mid level frontogenesis supporting the potential
for a narrow stripe of heavier amounts moving north from Northwest
Texas through central Oklahoma into southeastern Kansas overnight.
 The 12Z models showed a tightening of the western gradient along
with an easterly shift in the axis of accumulating snows across
this region. 

As the northern edge the precipitation shield continues to move
north on Friday, the potential for accumulating ice 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 at the onset, lifting north
across southern Iowa, central to northern Illinois, northern
Indiana and southern Lower Michigan during the day on Friday. 
These areas too should see a changeover to snow, with a few inches
possible before ending.  Most models agreed on a slight increase
in snow accumulations extending from southeastern Iowa to Lower
Michigan.

...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 the Eastern West
Virginia Panhandle and western Maryland into central Pennsylvania.

Day 2...
...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.

...Southern Great Lakes...
Precipitation is expected to continue across the region as low
pressure moves from the lower Ohio valley into the lower Great
Lakes Friday evening.  Additional snow is expected along the
western edge of the associated comma head as it moves across
northern Illinois and southern Wisconsin before ending Friday
evening, with a wintry mix continuing across southern and central
Lower Michigan before ending as snow Friday night.  Impacted areas
are likely to see no more than an additional inch or two of snow,
with up to a 0.1 inch of ice across portions of southern Lower
Michigan.  

...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 the
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 northern New
Hampshire and interior Maine, where snow is expected to remain the
predominate precipitation type through much of the event.  WPC
PWPF shows high probabilities for snow accumulations of 4-inches
or more across much of northern New Hampshire and central to
northern Maine.

Day 3...
...Pacific Northwest to the Northern Rockies...
A well-defined mid level shortwave and upper jet, accompanied by a
plume of deep moisture, will move into the region late Saturday. 
This will support additional heavy mountain snows from the
Olympics and northern Cascades to the northern and central Idaho,
and northwestern Montana ranges -- with accumulations of a foot or
more likely across a large portion of the Washington Cascades.

Pereira