Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
317 PM EDT Mon Oct 17 2022

Valid 00Z Tue Oct 18 2022 - 00Z Fri Oct 21 2022


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

The lake effect snow machine continues to generate bands of
moderate-to-heavy snowfall across Michigan's Upper Peninsula
tonight and into Tuesday. Persistent cold air advection over Lake
Superior continues to result in lake effect snow bands over
Michigan's Upper Peninsula. Periods of heavy snow within these
bands could be on the order of ~2"/hr at times. Latest WPC PWPF
indicates >4" snowfall probabilities are on the order of ~80% in
the heart of the U.P. with even some ~50% probabilities for >8" of
snow through 00Z Wed. Farther southeast, parts of northern
Michigan closer to Gaylord could also pick up >4" of snow. An
injection of positive vorticity advection rotating around the
northern flank of the large upper low over Lake Huron tonight acts
to enhance lift aloft while simultaneously, a surface trough
tracks south through the U.P. and on south through northern Lake
Michigan. This will produce a few hours worth of heavy snow,
primarily between 06-12Z Tuesday. Latest 12Z HREF 1"/hr snowfall
probabilities are as high as 40-50% closer to Gaylord.

There is a more intricate setup farther south over northern
Indiana and southwest Michigan. Current water vapor imagery shows
an upper level disturbance tracking south through northern Lake
Michigan. This feature will help induce supportive vertical ascent
atop the atmosphere. Meanwhile, strong cold air advection via NNW
flow over Lake Michigan will result in strong mesoscale forcing at
low levels. There is no shortage of instability over Lake
Michigan, so the main question comes down to boundary layer
conditions. Closer to the shores of Lake Michigan, there will be
too much marine influence for intense precipitation rates to
overcome warmer boundary layer temps, causing to precipitation to
fall as primarily rain. Farther inland, however, colder boundary
layer conditions combined with intense precipitation rates will
result in dynamic cooling aloft and allow for snow to become the
primary precipitation type. Strong vertical ascent within a
saturated DGZ will favor heavy, wet snowfall. CAMs remain at odds
on exact placement of the band, which in turn is making for
contrasting boundary layer conditions. Latest WPC PWPF shows a
narrow corridor of 20-30% probabilities of snowfall totals >4",
but given the robust mesoscale drivers in play, there is a
plausible scenario where localized areas see as much as 8" beneath
the most intense snowfall rates. The 12Z HREF contains 2"/hr
probabilities of ~30-40% in parts of north-central Indiana and
southwest Michigan between 00-12Z, which is the focus of WPC's
HREF Snowband Probability Tracker. Given the less supportive
boundary layer temperatures during the day on Tuesday flanking the
band, it will become more difficult to accumulate snow if a given
location is not beneath the primary lake effect band emanating out
of Lake Michigan. Intense snowfall rates of >2"/hr would allow for
quickly deteriorating travel conditions tonight and into Tuesday
morning.

Elsewhere, by late Tuesday into mid-week, temperatures aloft will
moderate over the Great Lakes, leading to a decreasing temperature
gradient between the lake temperatures and low level air
temperatures aloft. Light accumulations are possible across the
Michigan U.P., northern Michigan, and in the central Appalachians.
There is a slightly better chance for higher totals downwind of
Lake Erie in northwest Pennsylvania and south of Buffalo Tuesday
night, but the diminishing temperature gradient and lesser
instability values currently puts a cap on snow totals surpassing
4" for the time being.

The probability of significant icing is less than 10 percent.

Mullinax