Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
407 AM EST Sun Nov 13 2022

Valid 12Z Sun Nov 13 2022 - 12Z Wed Nov 16 2022

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

The lake effect snow machine will continue for one more day in
portions of the Michigan U.P., western Michigan, and across parts
of northeast Ohio, northwest Pennsylvania, and west-central New
York. Anomalously cold 850mb mb temps of -10C to -15C (which are
near the 10th climatological percentile Sunday morning and
afternoon according to NAEFS) will rush over the relatively warm
Great Lakes. The expectation is for delta-Ts that range between
15-20C which is quite suitable for lake effect snow bands to
develop. WPC probabilities do indicate some moderate values in
parts of Michigan's U.P. and in western New York, although
isolated totals surpassing 8" cannot be ruled out in these areas.
Lake effect snow bands should wind down overnight and into Monday
as high pressure builds in across the region.

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

A dynamic setup involving a vigorous upper level shortwave will
direct PVA at the region. 500mb height falls leads to a deepening
surface low in the Gulf of Maine and an associated 850mb low
tracking along the coast Sunday evening. To the north of the 850mb
low, a tightening 850mb frontogenesis field will act to focus a
narrow band of heavy snowfall to the 850mb front's west, which in
this case places the band over far northern Maine. Latest WPC
probabilities feature a 40-60% chance for >6" of snowfall and a
20-30% chance for >8" of snowfall in Maine's Aroostook county. The
WPC Snowband Probability Tracker utilizing the 00Z HREF does
contain some CAM members that could produce ~2"/hr snowfall rates.
As the surface low deepens over Quebec early Monday morning, heavy
snow could contain strong gusts as well, leading to near zero
visibility at times. Snow should begin to exit to the northeast by
mid-morning Monday.

...Southern Rockies, South-Central Plains to the Upper Midwest...
Days 2-3...

A pair of potent shortwave troughs will be the catalysts for
periods of snow to kick-off the upcoming week. Focusing in the
South, a small but intense 500mb disturbance tracking through the
Desert Southwest on Sunday will traverse the lower Four Corners
states Sunday night. An influx of 700mb moisture combined with
diffluent 250-500mb flow out in front of the trough will foster
period of heavy snow in the southern Rockies Sunday night into
Monday morning. WPC probabilities for >6" of snowfall are as high
as 40-60% in the Sangre de Cristo's of northern New Mexico through
Sunday night. By Monday morning, the upper low will track into
eastern New Mexico where it will then place its strong divergent
flow aloft over the south-central Plains. 500mb heights over
eastern New Mexico are as low as the 1-2.5% climatological
percentile according to NAEFS by 12Z Monday. A developing 850mb
low over the TX Panhandle will tap into rich 850mb moisture flux
that is advected north into Oklahoma and Kansas. Most area
averaged soundings in southwest Kansas and central Oklahoma
indicate the strongest vertical velocities will be around 500mb,
which coincides with where the DGZ is both located and fully
saturated between 15Z Mon - 00Z Tues. Latest HREF guidance shows
>1"/hr snowfall probabilities as high as 70-80% in west-central
Oklahoma through mid-morning with ~40-60% probabilities in
south-central Oklahoma through midday. WPC probabilities also
shows a ~10-20% chance for snowfall totals >4" on Day 2 in parts
of the northern most Texas Panhandle and western Oklahoma, as well
as into portions of the Arkansas Ozarks.

Farther north, an upper low over south-central Canada and the
aforementioned upper trough over the south-central Plains will
help to generate a pair of surface lows; one in the Middle
Mississippi Valley and a strong one along the Gulf Coast. To the
north, 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 2 WPC probabilities for >6" of snow along the
coast of Minnesota's Arrowhead are up to 40-50% in some spots.
Even into Day 3, WPC probabilities for >4" of snow are still up to
70-80% thanks to easterly flow across Lake Superior bringing
moisture to the region. This will lead to a multi-day snow event
in the Arrowhead with 48-hour WPC probabilities for >12" of
snowfall up to 10-30%.

Southern Appalachians, Central Appalachians, Interior Northeast...
Day 3...

The upper level trough responsible for the heavy snow potential in
the south-central Plains 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 throughout Tuesday afternoon and
into Tuesday night. This setup favors a surface low tracing across
the Southeast Tuesday night and off the Mid-Atlantic coast by
early Wednesday morning. This is a complicated setup because while
this wave of low pressure becomes the primary low by Tuesday
night, there is a separate low in the Ohio Valley Tuesday
afternoon that weakens as it tracks north towards the Great Lakes.
With a dome of high pressure over southeast Canada, this leads to
a cold air damming (CAD) over the Northeast and Mid-Atlantic on
Tuesday.

There is enough low level cold in place, particularly in the
higher elevations above 1,000' AGL, to allow for precipitation to
fall as snow, but a warm nose via a southerly 850mb jet will lead
to a phase change to an icy wintry mix of sleet and/or freezing
rain. Currently, WPC probabilities for >2" of snow are highest in
the interior Northeast, or more specifically northern Pennsylvania
and into the northern Appalachians. Farther east, closer to the
Lower Hudson Valley on south through the Catskills, Poconos,
south-central Pennsylvania, and into the Potomac Highlands and
Blue Ridge Mountains, there is an opportunity for both minor snow
and ice accumulations. There remains a great deal of uncertainty
in phase type in these regions. At this time, WPC probabilities
for >0.10" freezing rain accumulations are highest in the Blue
Ridge of western North Carolina and into the Potomac Highlands.
While there remains a high degree of uncertainty in snow/ice
totals, it is worth noting the experimental PWSSI shows >20%
probabilities for "Minor" impacts from the central Appalachians to
the interior Northeast. For many locations, this would be their
first opportunity for measurable wintry precip this season.
Interests in these regions should continue to monitor the forecast
closely in the coming days. 

Mullinax