Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
249 PM EST Wed Nov 16 2022

Valid 00Z Thu Nov 17 2022 - 00Z Sun Nov 20 2022


...Northern to Central Rockies and nearby Front Range...
Days 1-2...

A strung out shortwave and associated vorticity lobe embedded
within large scale broad cyclonic flow will rotate southward out
of Canada tonight and continue to advect southward into the
Central Rockies by Friday. This feature will produce modest height
falls and PVA, with an accompanying upper level jet streak
providing additional large scale ascent. At the surface, this will
result in a cold front moving southeastward through late week,
with a wave of low pressure possibly developing along this
baroclinic gradient. As this feature pushes southward,
anti-cyclonic flow around a trailing Canadian high pressure will
drive E/NE flow into the terrain, and the overlap of this upslope
flow and increased frontogenesis will drive enhanced ascent into
terrain features to support periods of heavy snowfall. As the high
sinks into the region and heights lower aloft, the column will
continue to cool, driving a lower DGZ supporting high SLRs which
will yield efficient snow accumulation. 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. For D1, the heaviest snow is likely on
the upwind, or northeast, side of the Bighorn, Beartooth and
Absaroka ranges in MT where WPC probabilities for more than 6
inches of snow is above 70%. Other significant accumulations are
likely on D1 as far north as the Little Belt range and as far
south as the Park Range and Laramie Range in CO/WY where WPC
probabilities for more than 4 inches are 50-60%. By D2 as the
forcing shifts southward to focus more into CO, bringing
significant snow into the Front Range where WPC probabilities are
above 80% for 4+ inches and locally more than 10 inches is
possible. Additionally, with the cold column supporting snow into
the High Plains, a few inches of accumulation is also possible
along the I-25 urban corridor, including Denver, and out into the
Palmer Divide and Raton Mesa where WPC probabilities are low to
moderate for 2 or more inches of snowfall.


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

***Major lake effect snow event will begin tonight and last
through the upcoming weekend. Paralyzing snowfall possible
downwind of Lake Erie and Lake Ontario***

A long duration and heavy lake effect snow event will begin
tonight, and then ramp up impressively through Friday, with
paralyzing snowfall possible in some areas, especially downwind of
Lake Erie and Lake Ontario. The driver of this event will be a
longwave trough anchored across the eastern 2/3 of the CONUS,
which will be repeatedly reinforced by shortwaves rotating through
the flow across the Great Lakes. 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 +8C to +14C 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 LES this period.

For D1 /tonight and Thursday/ the event will begin to ramp up as
westerly shear develops across the lakes. This will focus the most
intense lake effect snow (LES) bands along the eastern shores
including the western U.P., Chautauqua Ridge, Tug Hill Plateau,
and much of western lower Michigan. The heaviest snowfall D1 is
likely 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 6 inches are above 80% in these areas,
with 12-18" or locally more possible in some locations. Additional
heavy snow is likely near Grand Rapids and Traverse City where
some longer fetch, or some effective fetch with moisture transport
from Lake Superior could enhance snowfall, but WPC probabilities
for more than 6 inches are generally 30-50%.

D2 is likely to be the crux of this event, and truly crippling
snowfall is possible where intense and nearly stationary single
bands setup along Lake Erie and Lake Ontario. As the 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
thunder, and snowfall rates of 3"/hr are expected. Since the shear
remains consistent through much of D2, this will likely feature
little wavering and a band of exceptional snow appears almost
certain. WPC probabilities for more than 6 inches are above 95% in
these bands, and even reach 30-50% for 18". Noted as well the 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 forecasting 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 on D3
as another trough swings through the area. Early D3 the LES will
remain impressive across Lakes Erie and Ontario, but should wane a
bit the latter half as a brief period of WAA occurs ahead of this
trough. Still, heavy snow is likely on D3 in these areas with WPC
probabilities indicating a high chance for more than 6", and an
additional 12"+ possible. Although outside of this forecast
period, 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 possible.
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:

--Confidence is increasing that several days of heavy lake effect
snow will occur east of the Great Lakes through the upcoming
weekend.

--Lake effect snow is expected to begin late Wednesday, with
periods of heavier snow continuing through Sunday. The most
intense snowfall will occur Thursday into Friday.

--Snowfall will at times likely be accompanied by lightning,
thunder, and rates exceeding 2"/hr. This will produce near zero
visibility and snow covered roadways making travel hazardous to
nearly impossible.

--Snow amounts in excess of 12 inches are likely in many locations
E/SE of the Great Lakes. The heaviest snowfall is expected in the
Buffalo, NY metro area and far northwest PA where several feet of
snow are possible.

--Very cold temperatures will accompany this event with high
temperatures possibly more than 20 degrees below normal by the
weekend.


Weiss