Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
355 PM EDT Wed Apr 06 2022

Valid 00Z Thu Apr 07 2022 - 00Z Sun Apr 10 2022

...Pacific Northwest...
Day 3...

On D3, a shortwave lifting onshore WA state will be on the front
edge of a 130kt Pacific jet streak and accompanied by confluent
flow south of the shortwave to drive ascent and moisture onshore.
This modest AR surging into the Pacific Northwest will surge PWs
to as much as +2 standard deviations above the climo mean, with
IVT slightly above normal as well according to NAEFS ensemble
tables. With the pronounced leading WAA, snow levels will be
generally 6000-7000 ft ahead of the cold front, but drop to around
3000 ft Saturday. While this will keep the heaviest snow above
pass level, where WPC probabilities for 6+ inches are above 80% in
the WA Cascades, as snow levels drop late D3 modest accumulations
of snow are possible at the passes including Snoqualmie and
Stevens after the changeover from rain.


...Northern Plains/Upper Midwest/Western Great Lakes...
Days 1-2...

Anomalous closed low progged to approach -3 standard deviations
compared to climo at 500mb will continue to spin slowly across the
Northern Plains and Upper Midwest into Friday before filling and
opening while ejecting into Canada Saturday. This closed low will
be repeatedly reinforced by spokes of vorticity swinging through
the trough, and interact with at least subtle diffluence within
the LFQ of a downstream intensifying jet streak. These features
together will drive a slow moving surface low pressure from WI
through MI D1 into D2 before weakening into a trough.

Precipitation around this low will be driven by three primary
mechanisms. The first, on D1, will be associated with impressive
WAA on an 850mb LLJ reaching 40 kts emerging from the Gulf of
Mexico. This robust and moist ascent will surge a theta-e ridge
into the region, upon which the deep layer ascent will produce
precipitation, which will fall as snow where the thermals are
supportive. For D1, this is likely in a region near Duluth, MN and
into northern WI and the western U.P. of MI where WPC
probabilities for more than 4 inches are as high as 70% just east
of the Bayfields Peninsula. The second driver of wintry precip,
also on D1, is where the theta-e ridge surges cyclonically into a
TROWAL on the WCB, and interacts with a modest deformation axis NW
of the surface low. The guidance continues to struggle with how
impressive the ascent will be within this deformation, but the
presence of the TROWAL suggests at least enhanced ascent and a
moistening column, so heavy snow is possible D1 within this
region. Where the band sets up, which is most likely from
northeast SD through far northwest MN, WPC probabilities for more
than 4 inches are above 50%, highest in the higher terrain
including the Coteau des Prairies and the Pembina Escarpment.

The final driver of heavy snow will occur on D2 as the low pulls
southeast into MI. As this occurs, robust CAA on northerly winds
will converge over Lake Superior and drive robust LES along the
south shore, including the Porcupine and Huron Mountains, as well
as the Keweenaw Peninsula. The LES should be short lived and wind
down by the end of D2 /Friday evening/, but WPC probabilities for
more than 4 inches are above 80%, and more than 8 inches is
possible in local maxima.


For Days 1-3, the probability of significant icing of at least
0.25" is less than 10 percent.

Weiss