Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
403 AM EDT Fri Apr 28 2023

Valid 12Z Fri Apr 28 2023 - 12Z Mon May 01 2023

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

Challenging forecast developing for the potential for heavy snow
accumulations in the Arrowhead of MN, northern WI, and the western
U.P. of MI through the weekend.

A complex series of shortwaves spread across central Canada and
into the Great Lakes will gradually merge into a strong closed
mid-level low centered over WI by Saturday night. The primary
driver of this evolution will be a robust shortwave and
accompanying vorticity lobe streaking southward out of
Saskatchewan to help organize other impulses into this closed
gyre. This closed low is progged then to continue to deepen into
an impressive trough, noted by 700-500mb height anomalies falling
to around -4 standard deviations according to the NAEFS ensemble
tables. This will become an expansive trough across the east,
bringing well below normal temperatures to the region, with ascent
being driven by height falls, PVA, and periodic upper diffluence
as jet streaks rotate around the deepening trough. At the surface,
this will result in a wave of low pressure tracking slowly
northeast from the Upper Midwest into the Great Lakes as a cold
front drifts eastward.

The result of this will be a long duration precipitation event,
developing first along a strengthening deformation axis which will
pivot near Lake Superior, aided by intensifying 850-700mb fgen
which will drive potent ascent into the DGZ beginning early on D2.
Some negative theta-e lapse rates collocated with this fgen band
should produce a burst of heavy precipitation, likely falling as
snow through dynamic cooling in the marginally favorable thermal
structure of the column. Late D2 into D3, the forcing for wintry
precipitation expands as moisture rotates around the stacked
system, with waves of moderate to heavy precipitation wrapping
around the western side of the low within the most robust
deformation and being aided by Lake Superior moisture. SLRs will
likely be quite low as it is late April, and precip rates may
dictate precip type with some mixing with rain at times. However,
where precipitation falls heavily, moderate to heavy accumulations
of snow are becoming more likely. Still a lot of uncertainty at
this range due to mixing, the potential for an intense band
pivoting somewhere across the area, and the concern about a lack
of intense LES (850-lake delta-T is just around 10C), but a long
duration moderate to heavy snow event is becoming more likely.
Current WPC probabilities are high for more than 4 inches both D2
and D3 focused around the Porcupine Mountains of the western U.P.
of MI, but some of this will be dependent on where the band pivots
on Saturday/Saturday night. Locally more than 12 inches of snow is
possible in far northern WI or the western U.P.

The probability of significant icing greater than 0.25" is less
than 10 percent.


Weiss