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

Valid 00Z Sat Apr 29 2023 - 00Z Tue May 02 2023

...Western Great Lakes...
Days 1-3...

The Great Lakes region (in actuality, much of North America) will
be impacted by an increasingly amplified jet stream pattern. NAEFS
at 18Z Saturday shows a 97.5 climatological percentile ridge
forming over western Canada which then carves out an impressive
500mb low over the Great Lakes on Sunday. Between 12Z Sunday - 12Z
Monday, NAEFS shows 850-500mb heights that are outside the lowest
observed heights in thew CFSR climatological record for late
April-early May. This highly anomalous upper low over the Great
Lakes will remain stalled over the region through Monday evening
due to the developing high-latitude blocking pattern taking shape
over Greenland and northern Quebec. This prolonged period of
abnormally cooler temperatures and moisture revolving around the
upper low is a recipe for a multi-day snow event in portions of
the western Michigan U.P. and far northern Wisconsin.

From a synoptic-to-mesoscale evolution, the heaviest snowfall
starts Saturday night and into Sunday when the strongest upper
level divergence ensues as the upper mean 700-300mb trough takes
on a negative tilt. The GFS shows a zone of excellent 700-300mb
layer averaged Q-vector convergence directly over the western U.P.
of Michigan and northern WI that persists for the better part of
24-30 hours. As the 700mb low forms north of Green Bay Saturday
night, the warm conveyor belt at 850-700mb will also introduce a
ribbon of moisture flux overhead that persists into Sunday. By
Sunday night, a pivoting shortwave trough on the northwest flank
of the upper low will provide another source of positive vorticity
advection into the Michigan U.P. and northern Wisconsin, further
prolonging the heavy snow potential. The key in accumulations will
come down to thermals, vertical velocities in the DGZ, and timing
of heaviest snowfall rates. SLRs will be lower than climatology
for the most part, making for a heavy/wet snow. Taking GFS
cross-sections of the western Great Lakes Sunday morning and
afternoon, the strongest VVs were focused at the low levels where
there is added topographical/lake enhancement, but there was
strong 700-500mb frontogenesis as well, showing the strong
Q-vector convergence aloft is likely to be a key contributor to
heavier snowfall rates than just mesoscale lake effect/low level
convergence mesoscale effects.

The latest experimental PWSSI does show 40-50% probabilities of
Moderate Impacts along the MI/WI border near Ironwood. The
footprint of Minor Impacts ranging between 40-60% ranges from
northern WI to southeast of the Michigan U.P.'s Porcupine
Mountains. Confidence is increasing these areas picking up >12" of
snowfall over the course of the next 2-3 days. WPC probabilities
for 72-hour snowfall totals >12" were an impressive 70% near and
east of Ironwood. It is worth noting that probabilities for >18"
were up to 50% between this evening and Monday evening. Localized
impacts will be largely driven by the combination of Snow Amount
and Snow Load, which given the time of year and heavy/wet type of
snow expected, could lead to localized Major impacts where
snowfall totals surpass 12".

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

Mullinax