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

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

...Eastern Iowa through Michigan...
Day 1...

Heavy, wet snowfall is expected to push east from eastern IA
across southern WI and over across northern Lower Michigan. The
upper level shortwave is assuming a negative tilt as it moves from
the mid Mississippi valley toward southern MI this morning. A
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 00Z HREF
members indicating snowfall rates of over an inch/hr through the
mid-morning over eastern IA and persisting for through the
daylight hours across northern Lower Michigan. The latest WPC PWPF
has 50 to 75 percent probabilities for snow accumulations of
8-inches for much of the northern LP.


...Central Great Basin across the southern Rockies and the
southern Plains to the Ozarks...
Days 1-2...

A well-defined mid level shortwave trough/upper jet will continue
to dig southeast from OR the Great Basin to the Four Corners by
this evening before shifting east to AR through Sunday. Left-exit
region upper jet forcing and a strong low-to-mid level front
pushing southeast will support snow shifting rapidly through the
central Great Basin this morning to the southern Rockies later
today. While this system will be fast moving, several inches of
snow is possible across portions of the higher terrain. Day 1 Snow
probabilities are moderate for 6 or more inches from central NV
ranges to the southern Wasatch to the southern CO and northern NM
Rockies.

As this shortwave trough pushes east across the southern Rockies
tonight, lee side surface low cyclogenesis in the southern TX
Panhandle will promote bands of snow to shift east from southeast
CO and northeast NM across southwest KS and the OK Panhandle
before shifting to central OK Saturday morning. A period of strong
upper forcing along with low-to-mid level frontogenesis is
expected to support a swath of several inches of snow over the
southern High Plains. Day 1.5 snow probabilities are 30 to 60
percent for 6 or more inches over a swath of extreme southeast CO
southwest KS and the OK Panhandle. As the low encounters
warmer/moister conditions farther east, the snow risk decreases
with Day 2 snow probabilities low for 2 or more inches over
northeast OK and the Ozarks of northwest AR and southwest MO.

...Northern New England...
Day 1...

Warm advection precipitation east of the low tracking on on the
north side of the St. Lawrence River will allow a wintry mix
across portions of northern 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 and raise the potential for
significant ice accumulations across portions of northern New
Hampshire and western Maine. Day 1 ice probabilities are 10 to 20
percent for a quarter inch of ice there. Any heavy snow will be
contained in the cold sector which scrapes northern Maine where
Day 1 snow probabilities are moderately high for 4 or more inches.


...Pacific Northwest and Northern California to the Northern and
Central Rockies...
Days 2-3...

Mid level energy/an upper jet moving south of an upper low
centered over the Gulf of Alaska will shift into the West Coast
north from northern CA Saturday night before digging southeast
across the Great Basin Sunday night and reaching southern NM by
Monday night. Decent Pacific moisture will accompany this system
with heavy mountain snow expected. Day 2 snow probabilities are
moderately high for 8 or more inches for the higher Cascades and
northern Sierra Nevada. Trapped cold air will create some
potential for accumulating ice near the Columbia Gorge where Day
1.5 probabilities for a tenth inch of ice are 30 to 40 percent.
Mountain snows shift east to eastern OR, northern ID and northern
NV/UT with Day 2.5 snow probabilities moderate for 6 or more
inches for the higher Blue Mtns of OR, the Salmon River Mtns of
ID, the Independence and Ruby Mtns of northern NV, and the
northern Wasatch of UT.


...Central Appalachians...
Day 2.5...
An area of low pressure re-developing in the Southeast ahead of
the shortwave trough shifting east over the southern Plains on
Sunday is expected to lift northeast over the Carolinas Sunday
night into Monday. Some mountain snows are expected on the back
side of the precip shield, but as of now Day 2.5 snow
probabilities are only 20 to 30 percent for two or more inches of
snow for the Allegheny Mtns of WV/MD/southwest PA.

Jackson