Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
610 PM EST Thu Nov 17 2022

Valid 00Z Fri Nov 18 2022 - 00Z Mon Nov 21 2022


...Central Rockies...
Day 1...

A shortwave trough rotating through broad cyclonic flow across the
CONUS will drop across the Central Rockies tonight and Friday,
accompanied by a fast moving upper jet streak which will pivot
into the Central Plains. This will result in an overlap of ascent
driven by height falls, PVA, and diffluence within the RRQ of the
upper jet. At the same time, a wave of low pressure digging
southward will pull a cold front into the region, leaving NE winds
in its wake to favorably upslope into the terrain of CO and WY.
This will be in addition to enhanced mid-level fgen in response to
this baroclinic gradient, and aided by the ageostrophic response
of the upper jet moving overhead. Together, these should yield an
expansion and intensification of snowfall tonight, with a focus
likely occurring along the Front Range from the Cheyenne Ridge
through the Palmer Divide, and back into the Park Range. Snowfall
rates of 0.5-1"/hr may occur as some transient bands may develop
within the better fgen aided by upslope UVVs, especially in the
cold column which could support high SLRs. This heavy snow will
likely include the I-25 urban corridor, including Denver, but the
highest accumulations should be in the foothills and higher
terrain. WPC probabilities for more than 4 inches are above 50% in
the CO Front Range, but snow should wind down the latter half of
the day.


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

***Paralyzing lake effect snowfall (LES) likely downwind of Lakes
Erie and Ontario with heavy LES likely east of the other lakes***

A long duration, impressive, and widespread lake effect snow event
has begun and will continue to ramp up through Friday, producing
paralyzing snowfall in some areas. The overall model guidance
continues to suggest ideal LES to the east of Lakes Erie and
Ontario, with heavy snow also occurring in the western U.P. near
the Porcupine Mountains, and within the Tip of the Mitt in lower
Michigan near Traverse City and points north. Strong CAA will drop
850mb temps to as low as -20C by Saturday, which will produce
deltaT values of 25-30C due to lake temperatures that are +6 to
+13C according to GLERL. This will produce extreme lake-induced
instability nearing 2500 J/kg, with inversion heights of 10-15
kft, supporting the potential for thunder and lightning within the
most intense snow bands. Shear aligned with the length of Lake
Erie indicates the heaviest snow rates will be near Buffalo, NY,
but some effective fetch tapping Erie moisture and surging into
Lake Ontario will also drive intense snowfall near Watertown, NY.
Additional heavy snow rates are likely in the western U.P. and
northern L.P. where high instability and enhanced convergence
could result in multiple mesolows to further enhance omega, and
again an effective fetch from Lake Superior could enhance moisture
to produce efficient snowfall in the northern lower peninsula as
well. Finally, heavy snow is also likely along the western edge of
the L.P., with another maxima in snowfall likely in SW MI near
Grand Rapids due to a longer fetch for parcels on westerly shear.

While periodic shortwaves will cause some variation in wind
directions which will affect both the placement and intensity of
snowfall, it is likely that through the next 3 days many areas
east of the Lakes will see impressive snowfall. The WPC snowband
tool is highlighting many areas with 2+"/hr rates, and with SLRs
likely nearing 20:1, where convection can aid in ascent, snowfall
rates may reach 4"/hr at times. This is most likely downwind of
Lake Erie and Lake Ontario, and the recent HREF and WSE guidance
continues to increase the snowfall forecast. 3-day snowfall will
likely eclipse 3 feet in many areas within the bands, and expect
local totals exceeding 50" where the bands persist the longest.
This is most likely near Buffalo, NY, but Watertown, NY could also
receive this extreme snowfall. At least low probabilities for more
than 12 inches of snow exist each day downwind of these two lakes,
but are highest D1 when locally 2-4 ft or more is possible near
Buffalo and Watertown. Additional snow of 1-2 ft is possible D2 in
similar locations to D1, before the shear shifts to be more
westerly producing a high risk for more than 6 inches back into
the Cleveland, OH area, as well as along the Chautauqua Ridge and
Tug Hill Plateau.

Farther to the west, heavy LES is also favored into the western
U.P. through the Keweenaw Peninsula, and along the entire stretch
of the western L.P. Some of the snowfall here will be dependent on
placement of mesolows that are progged by the high res guidance to
enhance snowfall, but rates of 1-2"/hr are likely, potentially
higher at times. While the snowfall off Lakes Michigan and
Superior is not expected to be as extreme as to the east,
persistent LES with fluctuations in intensity should still produce
1-2 feet in the favored belts, with locally higher amounts
possible. Although fluctuations in wind direction driven by
shortwaves aloft will allow the bands across Upper and Lower
Michigan to fluctuate a bit more, there continues to be a high
probability for more than 6 inches of snow Friday and Saturday
across the eastern shores, with a shift to just the eastern U.P.
and northern L.P. D3 as the winds become more northerly. Local
event totals will likely eclipse 2 feet in some areas, with the
highest risk just northeast of Traverse City, MI.

By Sunday the last shortwave trough is progged to move eastward,
followed by shortwave ridging in its wake. This should finally
bring a slow end to the LES east of Lakes Superior and Michigan,
but LES will likely continue east of Lakes Erie and Ontario until
early Monday.

Key Messages for Lake Effect Snow in the Great Lakes:

--Periods of heavy lake effect snow will persist through Sunday
downwind of each of the Great Lakes. The most intense snowfall is
expected through Friday.

--East of Lakes Erie and Ontario, snowfall will at times be
accompanied by lightning, gusty winds, and rates of 3+ in/hr. This
will produce near zero visibility, nearly impossible travel,
damage to infrastructure, and paralyze the hardest-hit communities.

--Across parts of the Upper Peninsula and western Lower Peninsula
of Michigan, snowfall rates will reach 1-2 in/hr and combine with
gusty winds to produce near zero visibility and dangerous travel.

--Total snow amounts of 1-2 feet are expected in many locations,
with 2-3 feet likely east of Lakes Erie and Ontario. Historic
snowfall exceeding 4 feet is possible around Buffalo, NY.

--Very cold air will accompany this event with temperatures 20
degrees below normal forecast by the weekend.



Weiss