Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
420 PM EST Mon Nov 28 2022

Valid 00Z Tue Nov 29 2022 - 00Z Fri Dec 02 2022

...Pacific Northwest, California, and Northern Rockies...
Days 1-3...

An active and wintry period is expected over the region through
this weekend from repeating waves coming off the North Pacific. A
reinforcing shortwave trough is currently digging south-southeast
from the Pacific Northwest and will reach Colorado tonight. Day 1
snow probabilities are are generally low to moderate for an
additional four inches over the Olympics and southern WA/OR
Cascades in continued onshore flow.

The next potent system drops down the British Columbia coast
Tuesday as a closed low, reaching the Pacific Northwest coast late
Tuesday night. A reinforcing shortwave trough Wednesday shears the
low into a positively-tilted trough that shifts down the Pacific
Northwest coast into. This cold-core low/trough will bring a
prolonged period of unsettled weather into Friday with initial
precip into western Washington Tuesday afternoon. Low snow levels
around onset will allow for accumulating snow down sea level in
western Washington late Tuesday into Wednesday morning where a 50
to 80 percent chance of 1" exists (including the Seattle metro)
before the warm air advection the precip is arriving on raises
snow levels.

At higher elevations and places with direct exposure to Pacific
moisture a heavy snow event for the mountain areas is expected to
begin late Tuesday in western WA and shift south down the coast
through the Cascades through Wednesday night, continuing in
northern California through Thursday. Moisture increasingly
reaches inland areas such as eastern Washington and Idaho as the
southwesterly jet downstream of the trough axis strengthens
Wednesday and Thursday. Day 2 snow probabilities for 8 or more
inches are high for the Olympics and Washington Cascades and
moderate for the ranges of north-central and northeast Washington
and the Oregon Cascades. Day 3 snow probabilities are high for 8
or more inches over the Oregon and California Cascades and the
northern Sierra Nevada and moderately high for the Sawtooth and
Bitterroot Ranges of ID/MT.


...Central Rockies, Central Plains through the Upper Midwest...
Days 1-2...


A digging trough over the West will induce lee cyclogenesis later
tonight/early Tuesday over eastern Colorado and as the trough
amplifies over the Plains, the associated low will quickly track
northeast toward the Upper Mississippi Valley through Tuesday
night. Day 1 snow probabilities are high for 8 or more inches over
the Wasatch of Utah, western and northern Colorado ranges with
moderate probabilities of 6 or more inches over southeast Wyoming
and the Nebraska Panhandle.

Increasing upper divergence and low level frontogenesis northwest
of the surface low will support a stripe of accumulating and
increasingly heavy snow from Nebraska through the Upper Peninsula
of Michigan. Day 1 snow probabilities are low for 6 or more inches
in northeast Nebraska, but increase to over 20 percent in
northwest Iowa and southwest Minnesota and over 30% near and north
of the MSP metro as the low deepens and taps into better moisture
racing north, a intense frontogenetical band. The Day 1.5 snow
probabilities for 6 or more inches moderately high across northern
Wisconsin through the western U.P. and the Keweenaw Peninsula. a
is likely to form across portions of central/eastern Minnesota
through northern Wisconsin and U.P. of Michigan. 12Z guidance was
in good agreement and heavier CAMs such as the ARW2 and HRRR were
considered for their more potent banding potential extending over
much of a southwest to northeast swath across Minnesota.


...Great Lakes...
Days 2-3...

In the wake of the strengthening low pressure system lifting north
across Lake Superior Tuesday night, a strong high pressure (1035+
mb) shifts east from the Mid-Mississippi Valley to the
Mid-Atlantic. The resulting pressure gradient and strong westerly
flow will support a period of strong lake effect snowfall to
spread east across the lakes Wednesday and Thursday. Day 2.5 WPC
snow probabilities for at least 4 inches moderately in favored
snow belt areas in the western U.P. and northwestern L.P. Day 3
probabilities for 6 or more inches are moderate to high around off
Lake Erie around the Chautauqua region in western New York and in
the Tug Hill off Lake Ontario.


For Days 1-3, the probability of icing exceeding 0.1 inch is less
than 10%.

Jackson