Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
419 PM EST Thu Dec 10 2020

Valid 00Z Fri Dec 11 2020 - 00Z Mon Dec 14 2020

Day 1...
...Southwest to the Southern and Central Rockies...

As it moves across the region, an upper level shortwave associated
with a weakening upper low will continue to support high elevation
snow from the Rim Country and White Mountains of Arizona to
northern New Mexico and Colorado through the evening into the
overnight hours.  Several inches of accumulation are likely across
the higher reaches of the San Juan and Sangre de Cristo mountains.
 Even as this initial wave moves east of the region, snows are
likely to continue across portions of western to central Colorado
and northern New Mexico on Friday as an amplifying northern stream
trough moves from the Great Basin into the Rockies.

...Pacific Northwest...
A shortwave trough moving onshore early in the period, followed by
a trailing upper low moving into the region on Friday, will bring
periods of mountain snow, with several inches possible across the
higher elevations of the Olympics and the southern Washington and
northern Oregon Cascades.

Days 1-2...
...Central Plains to the Upper Great Lakes...
As the previously noted initial shortwave ejects east of the
Rockies, precipitation will likely develop and spread east across
the central Plains by early Friday, and then continue through the
day as the amplifying northern stream trough continues to move
east.  Overall consensus of the models shows a swath of light
accumulating snows spreading east from eastern Colorado and across
southern Nebraska and northern Kansas.  By late Friday into Friday
night, strengthening low-to-mid frontogenesis may help to support
a stripe of heavier accumulations farther east from eastern
Nebraska through southern Iowa.  Snows will continue to spread
northeast across far northern Illinois and southern Wisconsin
Saturday morning.  By mid to late day, a coupled upper jet along
with strong low-to-mid level frontogenesis are expected to support
moderate to heavy snows on the northwest side of a deepening
surface low as it tracks from Illinois into Lower Michigan.  While
confidence in the details remains limited, several inches of wet
snow appears likely across portions of northern Lower Michigan --
with WPC PWPF indicating 50 percent probabilities for
accumulations of 4-inches or more.


Day 2...
...Oregon/California/Great Basin/Central Rockies...
An upper low moving into the Pacific Northwest on Friday is
forecast to move east-southeast across the northern Rockies on
Saturday.  Low-to-mid level frontogenesis on the southwest side of
the low, along with favorable upper jet forcing, will help support
widespread snows from the Oregon Cascades to the central Rockies. 
Given the progressive nature of the system, widespread heavy
amounts are not expected, however several inches are possible
across the higher elevations, including portions of the Oregon
Cascades, the northern and central Nevada mountains, the Wasatch,
Uintas, and the western Colorado ranges.  Several inches are also
like across portions of the Sierra as moist northwesterly flow
interacts with the front as it moves into the region on Saturday.

Day 3...
...Great Lakes/Northeast...
Snows will likely begin to diminish across northern Michigan as
low pressure begins to move east of the region Saturday night. 
Meanwhile, warm advection precipitation ahead of the system will
likely begin as a wintry across portions of northern New England
on Saturday, with several hours of frozen precipitation possible
across portions of northern New Hampshire and Maine as a weak
developing along the coast helps to keep the low level cold air in
place late Saturday into Sunday.  Latest WPC PWPF shows high
probabilities for measurable ice across portions of northern New
Hampshire and much of northern Maine, with a significant threat
for accumulations exceeding 0.10 inch over portions of northern
Maine.  Several inches of snow is also possible across far
northern Maine, where the precipitation type is most likely to
remain snow through the event.

...Central Rockies to the Southern Plains...
A well-defined shortwave southwest of the low over the northern
Rockies on Saturday is forecast dig southeast, carving out a
trough over the central Rockies into the central and southern
Plains by early Sunday.  This will support snows spreading east
from the central Rockies early Sunday, with low-to-mid level
frontogenesis and favorable upper jet forcing, along with an
influx of deeper moisture, raising the potential for perhaps a
stripe of heavier totals farther east across portions of eastern
Kansas, the Panhandle region and western Oklahoma Sunday morning.

...Pacific Northwest and California to the Northern Rockies...
Shortwave ridging in place late Saturday is forecast to give way
to mid level energy/an upper jet moving south of an upper low
centered over the Gulf of Alaska.  This will bring mountain snows
back into the region, but with any heavy totals likely confined to
the higher elevations of the Cascades.  Freezing rain, with some
accumulating ice will also be a concern across the portions of the
interior valleys of Washington and Oregon. 

Pereira