FOUS11 KWBC 010812
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
312 AM EST Sun Dec 1 2024

Valid 12Z Sun Dec 01 2024 - 12Z Wed Dec 04 2024

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

*** Heavy Lake-Effect Snow event into early Wednesday ***

Cyclonic flow underneath a longwave trough anchored over the
eastern half of North America will continue to funnel Arctic air
southeastward across all of the Great Lakes into the workweek.
Periodic disturbances in the northwesterly flow will occasionally
shift the lake-effect snowbands off their predominant northwest to
southeast orientation, giving some of the hardest hit areas a break
for a time. GLERL modeled lake surface temperatures range from
44F/7C in western Lake Superior to 52F/11C in portions of eastern 
Lake Erie and southern Lakes Michigan and Ontario. Meanwhile, 850
mb temperatures average around -10C for much of the period until
considerable warm advection kicks over lower Michigan Tuesday
night. Thus, lake-to 850 mb temperature differences remain at least
15C apart, and in many cases are different by more than 20C. This
well exceeds the minimum threshold for lake-effect of 13C.
Instability will therefore remain well in range for the lake-effect
bands to sustain themselves well into the upcoming workweek.

Despite continued advection of cold air, the atmosphere remains
quite moist across the Great Lakes, with even the smaller upper
lakes such as Lake Nipigon (north of Lake Superior) contributing
moisture in the form of lake-effect clouds to the overall
atmosphere. The aforementioned periodic disturbances will further
increase the moisture available for lake-effect through the week.
Further, the above normal lake surface temperatures will further
add moisture to the atmosphere. Thus, lack of moisture also will
not inhibit lake-effect formation.

Therefore, the dominant factors which will adjust lake-effect band
strength and orientation will be the passage of a couple clippers
and a low through the period. The clippers will locally enhance the
lake-effect downwind of the lakes, while causing lighter snow
outside of the lake-effect areas. The flow will remain largely
northwesterly...more northerly over Lake Superior, and more
westerly towards Lake Ontario. Mesolows and wind shifts will adjust
the lake-effect band position, while locally enhancing snowfall
rates. A polar high will track west of the Great Lakes which will
reinforce the cold air and shift the flow more northerly Monday
night into Tuesday. Once that high shifts east into the Virginias,
return southwesterly flow and warm advection will push the lake-
effect bands northeastward towards Buffalo and Watertown off of
Erie and Ontario respectively. The bands should weaken some by
Tuesday due to the weakening flow and diminishing lake-surface to
850 mb temperature difference. However, it's unlikely that even
the increasingly unfavorable environment will be able to kill off
the lake-effect entirely as lake-effect bands tend to be resilient
and maintain themselves well after the surrounding atmosphere
becomes less favorable.

Expect an additional 1-2 feet of snow for portions of the U.P. and
northwestern L.P. of Michigan through Tuesday night, with 2-3 feet
forecast between Erie and Buffalo and around Watertown. WPC 72-hour
PWPF has a 30-40% chance of at least 2 feet of snow for northern
Chautauqua County, NY and a 40-50% chance of at least 2 feet of 
snow for southern Jefferson County, NY through Tuesday afternoon.
Meanwhile the WSSI shows extreme impacts continue for some of the
Buffalo south towns right along the Lake Erie shoreline as well as
for the Thousand Islands region around Watertown through Tuesday
evening.

The ongoing lake-effect snow is the subject of the Key Message linked below.

...Southern IN/northern KY to West Virginia... Day 1...

A very weak surface low (1020 mb) is moving up the Ohio Valley this
morning. Narrow bands of heavier snow are weakening on radar 
across southern Indiana, meanwhile light snow shower activity 
is spreading eastward from there into the mountains of West 
Virginia. The Appalachians will absorb nearly all of the moisture 
with this system through this morning. Thus, the primary threat for
brief periods of moderate snow will be into the mountains of West 
Virginia as upslope support will locally increase snowfall 
intensity. WSSI values suggest minor impacts in southeastern West 
Virginia. WPC PWPF shows a 60-70% chance of 2 inches of snow or 
more for much of southern West Virginia, though chances for 4 
inches or more decrease to 10-20%.

...Upper Midwest/Upper Great Lakes... Day 3...

A more substantial clipper low will move across the Canadian
Prairies on Day 3/Tuesday. Associated snow will break out well
ahead of the low along a strong warm front into northern Minnesota
Tuesday morning, spreading southeastward across much of Michigan
and the Upper Great Lakes through Tuesday night. Since this clipper
will have better forcing to work with as compared to the current
clipper disturbance over the Ohio Valley, expect a better chance of
stationary or nearly stationary heavy snow bands to set up along
the front in the aforementioned areas. Further, with conditions
still favorable for lake-effect, expect lake-enhancement southeast
of the lakes in the favored areas of the U.P. and northwestern L.P.
The trailing cold front behind the low will reinforce the cold air
along the Canadian border with North Dakota and Minnesota. The
heaviest snow associated with the clipper will move over the
western U.P. and northeastern Wisconsin into Tuesday night.

The probability of significant ice across the CONUS is less than 
10 percent. 

Wegman

...Winter Storm Key Messages are in effect. Please see current 
 Key Messages below...

https://www.wpc.ncep.noaa.gov/key_messages/La...

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