Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
446 AM EST Thu Nov 10 2022

Valid 12Z Thu Nov 10 2022 - 12Z Sun Nov 13 2022

...Northern High Plains into Upper Mississippi River Valley and
Western Great Lakes...
Days 1-2...

The stage is set for the first major winter storm to unfold from
the northern High Plains to northern Minnesota today. A powerful
upper low directing PVA at the region, along with steady 850-700mb
WAA out ahead of the upper low, will lead to robust vertical
motion within a deep and saturated DGZ. Meanwhile, very cold
temperatures aloft and at the surface allows for high SLRs that
are in some cases as high as 20:1. The focus for heavy snow will
become positioned over North Dakota where a warm conveyor belt of
rich 850-700mb moisture will become oriented along the northern
flank of a deepening 700mb low tracking along the ND/SD border.
This also coincides with the best 700-300mb Q-vector convergence,
focusing the strongest upper level ascent over the heart of North
Dakota. Guidance has come into agreement on central and eastern
North Dakota, in addition to northwest Minnesota, as being the
most heavily impacted as the deformation zone pivots over these
regions. Latest 00Z HREF Snowband Probability Tracker indicates
the aforementioned deformation zone could contain hourly snowfall
rates as high as 2-2.5"/hr in some cases. Latest Day 1 WPC
probabilities feature a 60-80% chance for >12" snowfall
accumulations from Bismarck on northeast into northwest Minnesota.
There are even some 30-50% probabilities for snowfall >18" near,
and just north and west of, Grand Forks, North Dakota. In
addition, a tightening pressure gradient promotes blustery winds
to ensue across the Dakotas and northern Minnesota. Wind gusts
will be strong in some cases within the deformation zone. The
11/09 run of the ECMWF EFI showed 0.7-0.9 values for wind speeds
and a Shit of Tails area highlighted near the Red River of the
North, implying potentially significant winds are possible here
that will only make it tougher on travel and infrastructure. The
latest WPC experimental PWSSI showed a ~60% chance of experiencing
"Major" impacts on Day 1 from central North Dakota to northwest
Minnesota. The PWSSI is largely weighted on snow amount, snow
rate, and blowing snow.

In terms of ice, the bulk of the heaviest ice accumulation shift
from eastern South Dakota to northern Minnesota. The recent 00Z
HREF showed 1-hour probabilities of experiencing freezing rain
that in some cases were as long as 10-12 hours in duration. With
sub-freezing surface temperatures locked in courtesy of the large
"banana" high pressure stretching from south-central Canada to the
northern High Plains, the warm nose of above freezing temperatures
at low levels will support multiple hours of hazardous freezing
rain and occasionally sleet as well. Latest WPC probabilities
showed as high as a ~50% chance for freezing rain accumulations
greater than a tenth of an inch in the Minnesota Arrowhead on Day
1. There is even a small 10-20% chance for greater than a quarter
of an inch in the Arrowhead, which would put more strain on
infrastructure should totals eclipse a quarter-inch.

The storm system will weaken as it tracks north of the Great Lakes
Thursday night. Periods of snow and gusty winds will linger into
Day 2 across northern Minnesota where the backside of the
weakening occluded low will reside before lifting northeast into
Ontario Friday evening.

...Great Lakes...
Day 3...

A surface trough trailing the Northern Plains/Upper Mississippi
Valley and abnormally cold temperatures filtering down the from
Canada will lead to steep enough lapse rates to generate lake
effect snow bands off the Great Lakes in Day 3. Guidance has shown
some delta-Ts that range between 16-20 degrees, which combined
with northerly upslope enhancement along the U.P. will allow for
an opportunity for heavy snow bands to materialize at times. Lake
Superior will be the focus initially for the heaviest snowfall
totals with Day 3 WPC probabilities showing 50-80% chances for >6"
of snow. Should lake effect bands stall over a given area, totals
could easily approach a foot in some locations on Saturday. Lake
effect bands are also expected to develop over western Michigan
and downwind of Lakes Erie and Ontario late Saturday, but it is
unclear just how heavy and intense these particular bands will be
at this time. That said, some light snow accumulation is possible
in these areas Saturday night.

Key Messages for the Northern Plains and Upper Mississippi River
Valley:

-- A winter storm will generate heavy snow, as well as sleet and
freezing rain, in portions of the northern High Plains, Great
Plains, and Upper Mississippi River Valley through Friday.

-- Heavy snowfall rates of 1-2"/hr are possible today from the
Dakotas eastward into northwestern Minnesota. Total snowfall will
likely exceed 12" in some areas.

-- Blizzard conditions are expected across portions of the central
Dakotas and northwestern Minnesota due to strong winds and reduced
visibility from blowing snow. Travel may become dangerous in some
areas and scattered power outages are possible.

-- Freezing rain and sleet are likely to the south of the heaviest
snow. Icing may be significant. This may produce hazardous travel,
disruptions to infrastructure, and scattered power outages.

-- Strong northeasterly winds over western Lake Superior may lead
to hazardous waves.

Mullinax