Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
318 PM EST Tue Dec 10 2024

Valid 00Z Wed Dec 11 2024 - 00Z Sat Dec 14 2024

...Northeast...
Day 1...

Broad southwest flow ahead of an upper low dropping south through
Manitoba and a southern stream shortwave moving across the central
and southern Plains will override a ridge of high pressure from
Atlantic Canada, keeping sub-freezing temperatures in place at the
surface across much of central and northern New England. A wavy
frontal boundary will attempt to lift northward into the cold air,
but will be slow to make significant progress until tomorrow
(Wednesday). As precipitation moves into the region tonight and
tomorrow morning, freezing rain is expected to spread across
portions of central and northern New England, especially in areas
of higher elevation. As the day progresses, southerly to
southwesterly flow will prevail, changing precipitation to rain.
Ice accumulations of 0.05-0.10 inch, with locally heavier amounts,
are expected in the mountains of northern New England, especially
from the White Mountains eastward into northern Maine. WPC
probabilities for 0.10 inch or more have come down, however
probabilities for accumulating ice (0.01 inch) remain above 70
percent across this area.

...Central Appalachians...
Days 1-2...

The southern part of the system affecting the Northeast tonight
will evolve into a deep, negatively-tilted trough over the eastern
CONUS on Wednesday. As the associated frontal boundary moves east
of the Appalachians, colder air will fill in behind it, changing
rain to snow from the Cumberland Plateau to areas north and east.
Upslope enhancement in the central Appalachians will support modest
snow totals, with WPC probabilities of greater than 30 percent for
accumulations of 4 inches or more largely confined to eastern West
Virginia.

...Great Lakes/Northeast...
Days 1-3...

Driven by falling 500mb heights and a negatively-tilted trough,
lake effect snow will develop overnight and intensify on Wednesday
across the Upper Peninsula of Michigan and into western portions
of Lower Michigan. 850mb temperatures will drop to -15C and -25C,
supporting significant lake-to-850 DeltaTs. Snow will continue
through Thursday as winds gradually turn from the northwest to
west-northwest, as the upper low dropping south through central
Canada pivots and moves east of the Great Lakes.

In the Northeast, once the front clears the region and temperatures
drop below freezing, snow will increase downwind of lakes Erie and
Ontario. Models continue to advertise intense, single-bands
targeting both the Tug Hill and the Buffalo Southtowns beginning
late Wednesday and continuing through Thursday. Some snow is
expected to continue into early Friday, but wane by late in the day
as a surface ridge builds over the region.

Through 00Z Saturday, WPC probabilities for at least 12 inches of
snow are greater than 70 percent across the favored lake-effect
snow belts of Upper Michigan, northwestern Lower Michigan, the
I-90 corridor from Erie to the Buffalo Southtowns, and the Tug Hill
Plateau. For some of these areas, especially those in New York,
WPC probabilities indicate that localized heavier amounts of 2 feet
or more can be expected.

...Cascades/Sierra Nevada...
Days 2/3...

A mid-to-upper level shortwave/compact low will drop southeast
from the eastern Pacific and move onshore on Thursday. This will
spread precipitation from the Cascades south into the Sierra. Snow
levels will be around 3000ft to the north and 5000ft to the south,
with some decreases with the passage of the shortwave. Some
additional decreases are expected Thursday night before rebounding
late in the period as a ridge begins to build and precipitation
returns ahead of a low/trough amplifying over the eastern Pacific.
Through 00Z Saturday, WPC probabilities for snow totals of 8
inches or more are highest over the Oregon Cascades into the
Shasta-Cascade region and the northern Sierra Nevada.

Pereira/Fracasso