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

Valid 12Z Thu Nov 17 2022 - 12Z Sun Nov 20 2022


...Northern through Central Rockies onto the Great Plains...
Day 1...

A zonal upper trough axis will shift south from western Canada,
crossing the northern Rockies today. A leading NWly jet will back
to Wly today as it drops south behind the cold front.
Anti-cyclonic flow around a trailing Canadian high pressure will
drive E/NE flow into the northern Rockies, and the overlap of this
upslope flow and increased frontogenesis will drive enhanced
ascent into terrain features to support periods of heavy snowfall
with high SLRs. The duration of the best overlap of forcing and
moisture is somewhat transient, but periodic intense ascent into
the cold column will still result in significant accumulations.
Day 1 WPC probabilities for more than 4 inches of snow is 60-80%
for the Front Range down along the I-25 urban corridor, including
Denver.

Jet-induced banding has spread over the western half of Neb this
morning which will shift south over KS through the day. Local
rates of 1"/hr or more can be expected with these snow bands.


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

***Major lake effect snow event will last through the weekend.
Paralyzing snowfall likely downwind of Lakes Erie and Ontario***

A long duration and heavy lake effect snow event will ramp up
impressively through Friday, with paralyzing snowfall in some
areas, especially downwind of Lakes Erie and Ontario. The driver
of this event will be lobes of shortwave energy crossing the Great
Lakes along a longwave trough anchored across the eastern 2/3 of
the CONUS. This will produce periods of increased synoptic ascent
to enhance the already impressive mesoscale lift driven by strong
CAA atop the lakes. 850mb temps are forecast to fall to as low as
-12 to -15C on Friday, and even potentially as cold as -20C late
Saturday across northern portions of the Great Lakes. These cold
temperatures will move across lake water temperatures that are
still +6C to +12C according to GLERL, providing steep lapse rates
and impressive lake induced instability of more than 2000 J/kg
with inversion heights climbing above 10,000 ft. Together, this
provides a setup that, when shear is favorably oriented across the
long fetch of the lakes, will produce exceptional lake effect snow
(LES).

Low level flow becomes westerly across the Lakes today with the
most intense LES bands along the eastern shores including the
western U.P., Chautauqua Ridge, Tug Hill Plateau, and much of
western lower Michigan. Heavy snowfall is expected along the
Porcupine Mountains of MI where the convergence of winds and
increased instability may drive a mesolow in the vicinity, with
additional lift provided by moisture upsloping into the terrain,
as well as east of Lake Ontario and Erie where westerly flow
catches more of the long fetch of the lakes. WPC probabilities for
more than 8 inches are 40-80% in these areas, with 12-18" or
locally more possible in some locations.

Tonight into Friday night looks to be the crux of this event and
extreme snowfall is expected in intense and nearly stationary
single bands setup along Lake Erie and Lake Ontario. As an upper
trough axis swings eastward, shear will focus more to the W/SW,
ideal for a long single band across Lakes Erie, with this
providing an additional effective fetch into Lake Ontario, pushing
directly into both the Buffalo and Watertown areas. Instability
which may spike at 2500 J/kg atop the lakes will support lighting,
and snowfall rates of 3"/hr are expected. Since the shear remains
consistent through much of Friday, this will likely feature little
wavering and a band of exceptional snow appears almost certain.
WPC probabilities for more than 18 inches are now 40-70% in/near
Buffalo/Watertown. CIPS analogs for this event have a median of
18" near Buffalo and 15" near Watertown, lending more confidence
to an extreme event, which is supported by WSSI indicating extreme
impacts in these areas as well. Where these bands focus the
longest, multiple feet of snow is likely, and is it possible
Buffalo could approach their daily single day snowfall record
should things line up just right. While the most intense snowfall
D2 is expected E/NE of Erie and Ontario, additional heavy snow
will also occur again in the western U.P., and western portions of
the L.P. where WPC probabilities for 6+ inches are as high as 70%,
and some areas could receive an additional foot of snow Thursday
night and Friday.

Conditions across the Great Lakes will become more variable
Saturday/Saturday night as another upper trough swings through the
area. A brief period of WAA occurs ahead of this trough. Still,
heavy snow is still expected on the backing flow which should lift
LES bands north on the eastern Great Lakes. One more round of
heavy LES appears likely on Sunday as the aforementioned trough
swings eastward. Farther to the west, this trough will amplify the
CAA to drive a rejuvenation of LES into the U.P. and western L.P.,
with mesolows and effective moisture fetch tapping multiple lakes
again possible to enhance snowfall. WPC probabilities are highest
for more than 6" near the Tip of the Mitt in Michigan, where
another 12" is possible, but are also significant across much of
the western U.P. and southwest L.P.

Event total snowfall in the favored belts will likely be measured
in multiple feet, with local maxima of 4 feet or more likely.
Widespread 1-2 feet appears nearly certain in many areas before
this event winds down late Sunday.


Key Messages for Lake Effect Snow in the Great Lakes:

--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, difficult to impossible travel,
scattered damage to infrastructure, and paralyze the hardest-hit
communities.

--Snowfall rates will reach 1-2 in/hr along the eastern shores of
Lakes Superior and Michigan, which will combine with gusty winds
to produce near zero visibility and dangerous travel.

--Total snow amounts of 1-2 feet are expected in many locations.
Maximum snow accumulations are likely to exceed 4 feet in or near
Buffalo, NY.

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




Jackson