Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
425 AM EDT Mon Oct 30 2023

Valid 12Z Mon Oct 30 2023 - 12Z Thu Nov 02 2023

...Northern Maine...
Day 1...

Periods of snow will ensue across northern Maine on Monday as a
positively tilted upper trough over the Midwest provides plenty of
upper level divergence over the Northeast. Snow will envelope
northern New England Monday morning as a surface low organizes
along the Northeast coast. Hourly snowfall rates Monday morning
may approach 1"/hr at their peak over northern Maine. Latest WPC
probabilities for >4" of snowfall are topping out as high as
70-80%. Probabilities for >6" accumulations were up to 30%,
suggesting some localized totals could top 6" for the event. The
WSSI depicted Minor impacts across northern Maine, and more
specifically, the North Woods and into St. John Valley and Central
Aroostook. Minor Impacts imply there could be some inconveniences
to daily life and motorists should exercise caution while driving.
Snow will taper off throughout northern Maine by Monday evening.

...Great Lakes...
Days 1-2...

West-northwesterly flow over Lake Superior that coincides with
unusually cold temperatures in the 925-700mb layer (NAEFS shows
850mb and 700mb temperatures as low as the 10th climatological
percentile) will trigger lake effect snow bands over the Michigan
U.P., as well as over the "Tip of Michigan's Mitten" and the
state's western-most counties. WPC probabilities show the western
side of the Keweenaw Peninsula sporting the higher-end
probabilities (up to 70%) through Monday. As northerly winds
accelerate on the back side of an amplifying upper trough diving
south through the Upper Mississippi Valley Tuesday night, CAMs are
suggesting the potential for a potent lake effect snow band
emerging off the Lake of the Woods in the Northwest Angle of
Minnesota. WPC probabilities are as high as 50% for snowfall
totals >4" just south of the Lake of the Woods. The WSSI shows
Minor Impacts downwind of the Lake of the Woods and along the
northern shores of Michigan's U.P., indicating there could be some
dicey travel conditions in some areas.

The aforementioned upper trough diving south through the Upper
Mississippi Valley Monday night is an exceptionally potent
feature, sporting 200-500mb heights that, according to NAEFS, will
be as low as the 1st climatological percentile. The GFS also
showed a potent PVU lobe associated with the trough. The lapse
rates along the 850mb front could be come as steep as 8.0C in the
sfc-3km layer over North Dakota Monday afternoon. A narrow ribbon
of 850-700mb moisture flux on the eastern flank of the upper
trough, the GFS shows strong 700-300mb layer averaged Q-vector
convergence as the upper trough moves east, and frontogenesis at
850mb helping to enhance mesoscale lift are all indicators for
possible snow squalls. The CIPS Snow Squall Parameter is highest
over North Dakota Monday afternoon, advances south and east into
southern Minnesota and central Iowa overnight, and then becomes
reinvigorated over southern Wisconsin and northern Illinois
Tuesday morning. The potential snow squalls could advance as far
east as Chicagoland and perhaps even into the lower Great Lakes
Tuesday afternoon and evening.

There remains some time to diagnose CAMs over the next 24 hours to
better assess the potential for snow squalls, but note that snow
squalls can produce a combination of whipping wind gusts and heavy
snowfall rates that result in a rapid decline in visibility. It is
also worth noting this upper trough is likely to produce locally
heavy snow on the northern flank of the 500mb low. This could
focus heavy snowfall rates along the western counties of Michigan
on Tuesday. The 00Z HREF currently depict a high chance (70-90%)
for snowfall rates to surpass 1"/hr early Tuesday afternoon. WPC
probabilities for >4" of snowfall around 30% over Manistee
National Forest. The WSSI-P shows a swath of 20-30% odds of Minor
Impacts from central Wisconsin to the western shores of Michigan.

Mullinax