Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
432 AM EDT Tue Oct 31 2023

Valid 12Z Tue Oct 31 2023 - 12Z Fri Nov 03 2023

...Great Lakes...
Day 1...

A strong and compact upper low traversing the Great Lakes will be
responsible for triggering snow squalls and fostering additional
lake effect snow showers on Tuesday. A surface low and its
associated cold front will track south and east and act as a
trigger for a narrow line of snow squalls advancing through the
Upper Midwest this morning. Snow showers will then develop around
the Lower Great Lakes Tuesday afternoon and evening. As the 500mb
low tracks towards lower Lake Michigan around midday Tuesday, the
left-exit region of an 85kt jet streak at 500mb will be located
over western Michigan. This also coincides within a strong area of
300-700mb layer averaged Q-vector convergence and 500-700mb
frontogenesis, suggesting strong rising motion at mid-upper levels
of the atmosphere. Cross sections of CAMs depict strong omega
within the DGZ, while the combination of strong dynamic cooling in
the boundary layer and easterly winds off the land (not the lake)
create a favorable environment for heavy snowfall rates. 00Z HREF
sported 90% probabilities for 1"/hr snowfall rates around midday
and into the early afternoon, while there are even some 30%
probabilities for 3"/hr rates in that same span. Given the strong
mesoscale banding potential, localized snow fall totals surpassing
6" is possible in western Michigan and could lead to snow covered
roads, as well as significantly reduced visibilities within heavy
snow bands.

As the 700mb low makes its way southeast Tuesday afternoon,
northerly low level winds will race over Lake Michigan and prompt
the formation of lake effect snow showers Tuesday afternoon. The
00Z HREF did suggest up to a 40% chance for 1"/hr snowfall rates
on the backside on the NW-W flank of the 850mb low in southeast WI
and northwest IN, but it is worth noting the milder lake
temperatures could lead to some mixing or rain and/or graupel. A
similar banding scenario that western MI witnessed earlier in the
day could also occur downwind of Lake Erie in northeast OH and
northwest PA. Just like western MI, lake enhanced snow bands have
a good chance to generate >1"/hr snowfall rates. This is evident
on the 00Z HREF where probabilities for 1"/hr snowfall rates are
as high as 80-90% Tuesday night. Potential impacts include rapidly
reduced visibilities and quick snow accumulation on sidewalks and
roads.

As the upper low tracks south and east, occasional heavy snow
showers may develop over parts of southern WI and northern IL as
the combination of daytime heating and highly unusual cold temps
in the 500-700mb layers (NAEFS shows some temps that would
approach CFSR climatological minimum for late Oct - early Nov).
Snow squalls could develop in some cases, leading to rapid
declines in visibilities and potential travel disruptions. The
upper low will lose some of its punch by the time it reaches the
Appalachians early Wednesday morning, but upslope flow combined
with temperatures within the atmospheric column being well below
freezing should result in some minor snow accumulations in the
higher terrain of the Allegheny Mountains and both the Potomac and
Laurel Highlands.

Mullinax