FOUS11 KWBC 110710
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
210 AM EST Wed Dec 11 2024

Valid 12Z Wed Dec 11 2024 - 12Z Sat Dec 14 2024


...Central Appalachians...
Day 1...

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 but also northward into the Laurel Highlands.


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

Driven by falling 500mb heights and a negatively-tilted trough,
lake effect snow will intensify today across the Upper Peninsula of
Michigan and into western portions of Lower Michigan. 850mb
temperatures will drop to -15C and -25C (below the 5th percentile),
supporting significant lake-to-850 DeltaTs. Snow will continue
through Thursday while 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 later today and continuing through Thursday. Snow will
gradually wind down from west to east across the Great Lakes
starting late Thu/early Fri before finally ending early Saturday
over NY as a surface ridge builds over the region.

For the event (next 72 hours), 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. Hazardous conditions can be
expected in the more intense bands where snowfall rates may exceed
2 inches per hour on Thursday per the 00Z HREF.

...Cascades/Sierra Nevada...
Days 1-3...

After a recent quiet period, the pattern will become more active
in the West over the next few days. A shortwave/compact upper low
will move ashore on Thursday, with some precipitation out ahead of
it today/tonight over the OR Cascades and the northern CA ranges.
Snow levels will be around 3000ft to the north and 5000ft to the
south, decreasing 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.
That system will have a little more moisture and amplitude,
focusing the precipitation northward into the WA Cascades by
Friday. Greatest impacts will be at the mountain passes throughout
the region from the WA Cascades southward into the norther Sierra.
WPC probabilities for 3-day snow totals of 12 inches or more are
highest over the Oregon Cascades into the Shasta-Siskiyous/Trinity
region and the northern Sierra Nevada.

...Corn Belt...
Day 3...

Upper low exiting the Rockies on Friday will start to tap limited
moisture out of the Gulf, eventually promoting an area of WAA-
driven precipitation across the Corn Belt. Cold air mass in place
beneath a warming ~850mb layer will favor an area of mixed
precipitation (sleet/freezing rain) just south of some light snow
on the northern side where the column remains below freezing.
Amounts may be light, but any freezing rain could be hazardous. WPC
probabilities for at least 0.1" of ice accumulation are 10-30%
over IA through 12Z Saturday.


The probability of significant icing greater than 0.25 inches is
less than 10 percent across the CONUS this period.

Fracasso/Pereira

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