Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
357 PM EDT Tue Mar 29 2022

Valid 00Z Wed Mar 30 2022 - 00Z Sat Apr 02 2022

...Central and Southern Rockies...
Day 1...

Potent trough moving into the central High Plains this evening
will swing eastward into the Central Plains by Wednesday morning.
Confluent flow downstream of this trough axis will continue to
interact with a subtropical Pacific jet streak to enhance moisture
across the region, with impressive ascent through height falls and
LFQ diffluence driving rounds of precipitation into the Central
Rockies. Snow levels within the warm airmass will generally be
6000-8000 ft, highest south, with significant accumulations of
snow likely above these levels. Snow rates will drastically
decrease as upper-level forcing exits the region on Wednesday,
with lighter snow lingering into Wednesday night. WPC
probabilities for more than 6 inches of snowfall are above 70% in
the San Juans, Sangre De Cristos and Raton Mesa of NM, many of the
CO Rockies, including the Front Range. Lighter accumulations are
likely in the higher terrain of WY and across the Palmer Divide.


...Northern Plains through the Upper Midwest, Great Lakes, central
and northern Appalachians...
Days 1-3...

Complex mid-level pattern development will produce a long duration
winter weather event for the Great Lakes and Upper Midwest through
early Friday morning. Impactful winter weather is also possible
across a large area from the Dakotas through the central/northern
Appalachians.

Guidance continues to show relatively good agreement regarding the
two distinct shortwaves rounding the base of a larger scale trough
that will move from the Plains through the Great Lakes, with the
associated WAA out ahead of the trough spreading wintry
precipitation as far east as Maine. However, subtle differences
and recent trends lead to a decent amount of uncertainty remaining
even though the event will occur in less than 48 hours. The first
shortwave and associated moisture plume will emerge out of the
central Plains this evening and then shift rapidly northeast into
the Upper Midwest by Wednesday morning. An area of low pressure
will deepen slightly as it lifts northeast in response to modest
height falls and strengthening LFQ upper diffluence as an
anticyclonically curved jet streak blossoms downstream of the
mid-level longwave trough axis. This forcing will combine with
increasing moist advection out of the Gulf of Mexico, the result
of which will be a swath of snow and sleet from SD through MN and
the western U.P, with moderate freezing rain likely from eastern
MN through WI, the L.P. of MI, and southeast into the Laurel
Highlands of PA and western MD. This event will be fast moving,
but anomalous moisture will still allow for heavy accumulation
potential of snow and freezing rain. WPC probabilities for more
than 4 inches of snow D1 as high as 50-60% along the eastern ND
and SD border, with 20-30% probs extending across northern
Minnesota into the Arrowhead. Freezing rain exceeding 0.1" on D1
is most likely to occur in north-central WI and the L.P. of MI.
Additional freezing rain is likely into D2 across northern WI, the
U.P., and the northern L.P. as well, with storm total freezing
rain possibly reaching 0.25" in a few effected locations in MI.

As wave 1 pulls away to the northeast and weakens, renewed
shortwave energy digging into the mean trough will combine with
intensifying upper diffluence within an coupled jet structure to
drive a stronger wave of low pressure from the Southern Plains
northeast into the Great Lakes D2. This low will likely produce
more significant wintry precipitation, as impressive theta-e
ridging rotates cyclonically into a TROWAL over the Upper Midwest,
and overlaps with a potent deformation axis that now has good
model support. Where this axis pivots, snowfall rates in excess of
1"/hr are likely despite modest SLRs as theta-e lapse rates fall
below 0C suggesting CSI potential. While the models are in better
agreement that this deformation will occur, there still remains
significant longitudinal discrepancies, leading to lowered
confidence in where the heaviest snow will occur. There is also
concern that some of this area will be rain initially, and the
timing of the transition to snow as CAA strengthens behind the low
will have a significant impact on snowfall accumulations. At this
time the heaviest snow is most likely in a narrow corridor from
central WI through the western/central U.P. of MI. Most of the
heavy snow will occur during the overnight hours, which does
support accumulation within the heaviest snowbands even if warmer
ground temperatures/rain existed before the switch to snow. WPC
probabilities on D2 are as high as 80% for 4+ inches in the
western U.P., with additional heavy snow likely into D3 associated
with impressive LES off Lake Superior. Both the WSE and NBM have
maintained their snowfall for this area D2-3, and locally 12
inches of snow is possible where the best overlap of LES and
banded snow occur (most likely near and west of Marquette, MI).

Finally, on D3 as the aforementioned low pressure system exits
into SE Canada and a potent cold front swings through the
Northeast, lake effect snow showers and isolated snow squalls are
possible throughout the Great Lakes, Ohio Valley, and Northeast.
The typical upslope regions of western PA, NY, and the central
Appalachians will have the best chances for light accumulations.
Elsewhere, warm ground temperatures will make it difficult for
snow to accumulate, but visibility could still drop briefly under
the heaviest snow showers. The NAM and GFS snow squall parameter
shows this potential as lapse rates steepen during the day on
Friday.


Snell