Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
327 AM EST Mon Nov 14 2022

Valid 12Z Mon Nov 14 2022 - 12Z Thu Nov 17 2022

...South-Central Plains, Upper Mississippi Valley, and Upper Great
Lakes...
Days 1-2...

A rigorous upper-level low/shortwave trough ejecting into the
southern Plains this morning will spawn a developing 850mb low
over the TX Panhandle. This low will tap into rich 850mb moisture
flux that is advected north through Oklahoma into Kansas and
intersecting with a fresh injection of subfreezing temperatures
behind a cold front. SW to NE oriented snow bands with the
potential for 1-2"/hr snow rates are progged by the 00Z HREF
Monday morning across the OK Panhandle into west-central KS.
Farther east, intense 700 mb frontogenesis will aid in drastic
dynamic cooling under convective bands over west-central and parts
of south-central OK between 16z and 21z Monday. The snow band risk
increases with several 00z HREF members depicting 1-hr snowfall
greater than 1" over these parts of OK. WPC probabilities for at
least 4" of total snowfall have increased across west-central OK
to around 30-50%. The mid-level trough becomes less sharp and
heavy snowfall rates become less likely as it is absorbed into the
parent low this evening with an inverted trough developing up the
MS Valley from a Gulf Coast sfc low to a trough in MN, which is
east of the parent upper low. This makes for a broad snow snow
risk area Monday night from AR through MN with generally light
amounts (less than 4 inches) though the threat for continued
embedded moderate bands does persist from KS into MO. Locally, a
separate area of potentially moderate snowfall exists over
south-central MN and north-central IA today as an 850mb low
resides along the aforementioned mid-level trough. An enhanced
low-level jet is likely to moisten the atmosphere and provide
sufficient forcing within the 700 mb DGZ. Localized totals could
approach 4 inches as snow ratios range between 12-15:1.

Elsewhere, the parent upper low over south-central Canada/Red
River of the North will direct creation of the inverted trough
Monday night and a strong dome of high pressure over southern
Canada and the northeastern U.S. will cause an elongated fetch of
southeasterly-to-easterly flow across the Great Lakes. The initial
upper low will help to produce periods of snow in the Upper
Mississippi Valley and northern Plains Monday morning. At the same
time, ESE 850mb flow over the western half of Lake Superior will
upslope into the Minnesota Arrowhead and focus heavier snowfall
bands there. Day 1 WPC probabilities for >6" of snow are 40 to 80%
for particularly the northern half of the North Shore. Continued
snow expected there through Tuesday night continues to bring
48-hour WPC probabilities for >12" of snowfall up to 30-50%. There
are accumulating snow chances across the Upper Midwest under the
upper low Monday afternoon into Wednesday with some 10%
probabilities for 4 or more over eastern WI and near the eastern
WI/IL border, as well as parts of MI through Wednesday morning.


...Appalachian Chain, northern New England, and the Lower Great
Lakes...
Days 2-3...

The upper level trough responsible for the heavy snow potential in
the south-central Plains Monday morning will make its way through
the Mid-South Monday night. As it does, the divergent left-exit
region of a 250mb jet streak will become positioned over the
southern Appalachians at first, then work its way up the spine of
the Appalachians and the East Coast through Tuesday. This setup
favors a surface low tracking across the Southeast Tuesday night
and off the Mid-Atlantic coast by early Wednesday morning, which
along with a dome of high pressure over southeast Canada leads to
cold air damming (CAD) over the Mid-Atlantic and Northeast by
Tuesday.

The leading warm air advection precip lifting up from the
Southeast Monday night and across the Mid-Atlantic/central
Appalachians Tuesday looks to have a super-freezing warm nose soon
after precip onset at least south of the Mason-Dixon line. So the
question becomes more about rain vs sleet/freezing rain Tuesday
morning/midday. The setup looks good for radiational cooling
Monday night allowing freezing rain and a light glaze of ice
generally west from the Blue Ridge, where there are 20-40% Day 2
probs for over a tenth inch of ice along the crest of the
Appalachians from western NC through the Allegheny Highlands and
Potomac Highlands.

A reinforcing shortwave promotes further low level cyclogenesis
Tuesday night through Wednesday as the sfc low tracks northeast
from Cape Hatteras just off the Northeastern Seaboard. This
results in a developing Nor'easter with a rain/mix line just
interior to the Northeast coast and swaths of snow farther inland.
The day 3 freezing rain probs for a tenth inch or more are 10 to
30% over the Catskills to the south-central Hudson Valley and
Berkshires, where lingering CAD takes hold along with being within
the typical transition zone between rain and snow. Just to the
north, colder air and a healthy left exit region of a 250 mb jet
streak will promote a broad area of moderate to locally heavy snow
between the Adirondacks and northern Maine. There remains some
uncertainty regarding the strength of the sfc low and associated
shortwave trough, with weaker solutions (like the 00z GFS) keeping
the heavier axis of snowfall more narrow and throughout Maine. WPC
snopw probs for more than 6 inches of snow on Day 3 are between
40-70% from the northern Green and White Mts to northern Maine.

Following the passage of the Nor'easter, strong west winds as
strong high pressure builds into the central and southeastern U.S.
will begin to create proper fetch off Lake Erie. 850mb temps by
Thursday morning will be around -15C, but more westerly winds
opposed to west-southwest to start will likely make the
lake-effect snow more multi-band to start, as opposed to a few
long/intense bands. Still, WPC probs for more than 6 inches of
snow on day 3 are already between 20-30% over northeast OH and
northwest PA.

...Central Montana to northern Wyoming...
Days 1 and 3...

Strong northwesterly flow and embedded shortwaves swinging around
a parent upper-level low in south-central Canada will aid in
periods of heavy snow across mountain ranges and upslope areas to
the north across central Montana and northern Wyoming. The first
system is ongoing on Monday in response to a approaching
shortwave, where WPC probs of greater than 8 inches is between
10-40%. By Wednesday and Wednesday night, the next system
associated with a stronger longwave trough and arctic cold front
will spread snow into similar areas able to take advantage of the
upslope northerly flow, as well as more widespread light snow in
valley locations. Given the approaching airmass, snow ratios are
likely to range in the 15-20:1 range.

Snell