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

Valid 12Z Mon Nov 28 2022 - 12Z Thu Dec 01 2022

...Pacific Northwest through the Rockies into the High Plains...

Days 1-3...
An active and unsettled period is expected to prevail over the
region through at least midweek with a couple of shortwave troughs
passing through. Currently, a strong shortwave is noted in the
latest water vapor passing through portions of the Pacific
Northwest and slipping southeast through the Intermountain West
and northern Rockies. As this feature continues to dig over the
Intermountain West and Rockies through Tuesday, favorable forcing
for ascent provided by a meridional 130+ kt jet streak and
orographic ascent will help bring accumulating heavy snow early to
the Oregon Cascades followed the Great Basin, UT ranges and much
of the central Rockies from southern WY into CO. This is where the
latest WPC probabilities for at least 6 inches of snow are
highest, generally 70 percent or higher. Localized 24-hr totals of
8-12" are likely, particularly for the UT ranges and central
Rockies.

Another potent system drops southeast toward the Pacific Northwest
Tuesday into Thursday with a shortwave trough expected to close
off northwest of Washington State before eventually stretching out
as a wave of low pressure develops on the southern end. This will
bring a prolonged period of unsettled weather, initially across
the Washington Olympics and Cascades with heavy snow. Low snow
levels initially may even allow for accumulations down into the
lower valleys of Washington late Tuesday into early Wednesday
morning (40-60 percent of 1"), though how much remains uncertain
with some guidance considerably colder and wetter. Regardless, a
heavy snow event is likely for the mountain areas Tuesday into
Wednesday with the highest amounts over the Olympics (Tuesday) and
the Cascades (Tuesday and Wednesday). As the atmospheric river
sags southward, the heavy snow will spread into the northern
California ranges late Wednesday into early Thursday, and moisture
spilling over into eastern Washington and northern Idaho is likely
to result heavy snow of 8-12"+ for the Day 3 period as supported
by the moderate to high probabilities of at least 8" and the
highest peaks likely to see totals well over a foot.

...Central Plains and Upper Midwest into the western Great Lakes...

Day 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. Increasing upper divergence and low level frontogenesis
northwest of the surface low will support a stripe of accumulating
snow from Nebraska through the Upper Peninsula of Michigan.
Generally lighter accumulations are expected across the Plains,
but as the low deepens and taps into better moisture racing north,
a intense frontogenetical band is likely to form across portions
of central/eastern Minnesota through northern Wisconsin and U.P.
of Michigan. Model guidance continues to waffle a bit on the
strength and location of the low track (and associated fgen band).
The latest deterministic forecast from WPC did trend a bit wetter
and also a tad to the south and east. The deterministic forecast
lies well within the ensemble spread showing the greatest
probabilities of 4-6"+ likely across southern/east-central
Minnesota through Michigan U.P. (40-70 percent) with the best
probabilities for 6"+ across northern WI and U.P. Michigan (40-50
percent) given the likely enhancement off Lake Superior with the
strong cold air advection and northerly low-level flow.

...Great Lakes...

Day 3
In the wake of the strong low pressure system lifting north into
Canada (deepening further too), a strong high pressure (1035+ mb)
settles over the Mid-Mississippi Valley. The resulting pressure
gradient and strong westerly flow will support a period of strong
lake effect snowfall Wednesday. The latest WPC probabilities for
at least 4 inches are already moderate across the favored snow
belt areas downwind of the lakes, particularly off Lake Erie and
Ontario where accumulations greater than 6" are likely.

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

Taylor