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

Valid 12Z Tue Mar 29 2022 - 12Z Fri Apr 01 2022

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

Closed mid-level low moving across CA this morning will gradually
weaken today into an open trough an d shift into the Four Corners
and then 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 across the Southwest
and 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. WPC probabilities
for more than 6 inches of snowfall are above 70% in the San Juans,
White Mountains of AZ, Sangre De Cristos, and many of the CO
Rockies, including the Front Range. Lighter accumulations are
likely in the Mogollon Rim, the Uinta Range, the higher terrain of
WY, and across the Raton Mesa and Palmer Divide.


...Northern Plains through the Upper Midwest, Great Lakes,
Mid-Atlantic, and New England...
Days 1-3...

Complex mid-level pattern development will produce a long duration
winter weather event during the next 3 days. While the most
significant impacts and accumulations are likely in the Upper
Midwest/Great Lakes, impactful winter weather is possible across a
large area from North Dakota through Pennsylvania and Maine.

The guidance has come into at least subtly better agreement
tonight that two distinct waves of low pressure will move from the
Plains through the Great Lakes, with the associated WAA out ahead
of the lows spreading wintry precipitation as far east as Maine.
The first wave will emerge out of the lee of the Rockies this aftn
and then shift rapidly northeast into the Great Lakes by Wednesday
morning. This low 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. 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 20% in the Coteau des
Prairies, with freezing rain exceeding 0.1" most likely in
north-central WI and the L.P. of MI. As this system continues
northeast into D2, heavy snow on the NW side of the low will
continue in eastern SD and ND, and spread into the northern half
of MN including the Arrowhead, where WPC probabilities for 4+
inches reach above 40%. 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
locations.

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 D2-3. 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 late D2 into D3. 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 are still 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 southwest WI through the western U.P. of MI where WPC
probabilities on D2 are as high as 70% for 4+ inches, with
additional heavy snow likely across the U.P. D3, first with this
heavy snow band, and later with impressive LES off Lake Superior.
Both the WSE and NBM have increased 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.

Finally, as the entire system shifts northeast, WAA precipitation
into the terrain of VT/NH/ME may produce some light accumulations
of freezing rain, but WPC probabilities for more than 0.1" of
accretion are only 5-10%.


Weiss