Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
354 AM EST Fri Dec 11 2020

Valid 12Z Fri Dec 11 2020 - 12Z Mon Dec 14 2020

Days 1-2...
...Central Plains to the Upper Great Lakes...
What was a southern stream shortwave trough is now over the
south-central High Plains this morning with a reinforcing northern
stream trough digging southeast over the northern Great Basin
(currently over northern UT) will swing east across the
south-central Plains tonight and promote surface cyclogenesis over
MO/IL.  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 this evening,
strengthening low-to-mid frontogenesis may help to support a
stripe of heavier snow to develop from eastern Nebraska through
central Iowa. Snows will continue to spread northeast across
southern Wisconsin Saturday morning. By mid Saturday, 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. Confidence in the rain/snow line and mesoscale banding
has increased some with several inches of wet snow expected over
southern WI and more so across portions of northern Lower
Michigan. Day 1 WPC snow probabilities for 6 or more inches are
generally 10 to 30 percent in the stripe under the comma head
precip in eastern Neb and west-central IA, expanding across
central and northeast IA and increasing to 40 percent over
portions of southern WI for Day 1.5. Day 2 snow probabilities or 6
or more inches are 40 to 70 percent roughly northeast from
Ludington, MI.


Days 1-3...
...Oregon/California/Great Basin/Central Rockies/Southern Plains...
An upper, northern stream low moving south-southeast into the
Pacific Northwest today will dig southeast across the northern
Great Basin to the central Rockies through Saturday night - in a
similar track to the leading shortwave currently over northern UT.
However, this second trough will shift farther south, pushing
across northern TX on Sunday before shifting east across the
interior Southeast Sunday night/Monday. Ample Pacific moisture is
directed inland south of the low track. Low-to-mid level
frontogenesis on the southwest side of the low, along with
favorable upper jet forcing, will help support heavy mountain
snows from the OR/CA Cascades and northern Sierra Nevada to the
central Rockies. Given the progressive nature of the system, the
overall snow totals will be somewhat limited, though snow
probabilities for 8 or more inches are moderate in the OR Cascades
on Day 1, the northern Sierra Nevada, central Nevada ranges, the
Wasatch, and western Uintas on Day 1.5, expanding across the
western Colorado ranges on Day 2.

Low-to-mid level frontogenesis and favorable upper jet forcing,
along with an influx of deeper moisture from the Gulf of Mexico,
gives further confidence in the potential for a stripe of heavier
snow east from the CO Rockies and across the south-central Plains.
Day 2.5 snow probabilities are moderate for 4 or more inches from
southeast CO, southwest KS to western OK and the far northern TX
Panhandle.


Days 2-3...
...New England...
Warm advection precipitation ahead of the system will likely begin
as a wintry mix across portions of northern New England on
Saturday, with several hours of frozen/freezing 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. Day 2 ice probabilities
for more than a tenth inch are moderate over northern NH and
western Maine, with a low threat for a quarter inch in the White
Mtns of Maine.  It remains cold over far northern Maine where
several inches of snow are expected with Day 2.5 snow
probabilities for 6 or more inches moderate in northwest Aroostook
County.

...Pacific Northwest and California to the Northern Rockies...
Shortwave ridging in place over the Pacific Northwest late
Saturday will give way to mid-level energy/an upper jet moving
south of an upper low centered over the Gulf of Alaska. This will
bring heavy mountain snows back to the Northwest the region. Day 3
snow probabilities are moderate to high for 8 or more inches for
the WA/OR/CA Cascades and northern Sierra Nevada. Freezing rain,
with some accumulating ice will also be a concern across the
Columbia River Gorge and east into the Valley with Day 2.5 having
30 percent probabilities for a tenth inch of ice. 

Jackson