FOUS11 KWBC 111959
QPFHSD

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

Valid 00Z Thu Dec 12 2024 - 00Z Sun Dec 15 2024


...Great Lakes/Northeast/Central Appalachians...
Days 1-2...

Deep negatively-tilted trough as low as 510 dam (below the 1st
percentile) diving over the Great Lakes along with very cold
temperatures will continue to produce heavy lake-effect snow over
the Upper Peninsula of Michigan and into western portions of Lower
Michigan through Thursday. The eastern lakes (Erie and Ontario)
will also develop significant lake-effect snowbands tonight through
Thursday as the trough nears the region and a developing surface
low along a strong cold front swings across northern New England.
Models highlight intense, single snowbands targeting the Tug Hill
and Buffalo Southtowns. Pronounced lake-850 DeltaTs are anticipated
from -15C to -25C 850mb temperatures (below the 5th percentile
across lakes Erie and Michigan), with the lakes remaining ice-free
and surface water temperatures between 5C and 9C. Snow will
continue through Thursday while winds gradually turn from the
northwest to the west-northwest as the upper low swings eastward
over the Great Lakes and into southeast Canada. Snow will linger a
bit longer downwind of the eastern Lakes into early Friday until a
potent surface high moves directly overhead.

Total lake-effect snowfall probabilities from WPC over the next 48
hours are highest over the favored snowbelts of Upper Michigan,
northwestern Lower Michigan, the I-90 corridor from Erie to the
Buffalo Southtowns, and the Tug Hill Plateau. Chances for at least
12 inches of snow are high (>80%) in these areas. For some regions,
especially eastern New York and into Erie, PA, the potential exists
for over 2 feet of snow and snowfall rates over 2 inches per hour.
This would create hazardous to potentially impossible driving
conditions.

For the Northeast and central Appalachians, cold air infiltrating
behind an intense moisture plume riding up the East Coast tonight
will allow for a brief changeover to snow throughout interior
locations. The heaviest snow (up to 4 inches) is expected early in
the D1 period across the central Appalachians due to favorable
upslope flow into Thursday, with a few inches possible in the
higher terrain of northern New England.

...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 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 by Friday
night 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 a broader
area of precipitation northward from the northern CA ranges into
the WA Cascades by Friday, with the heaviest precipitation expected
over northern CA Fri-Sat associated with an Atmospheric River. IVT
values are expected to peak around 700 kg/m*s over northern CA,
which will drive snow levels above 4500ft (over 5500ft in the
Sierra). 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, with the northern CA
potential seeing the greatest impacts from high elevation heavy wet
snowfall.

...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 20-50%
over IA through Saturday night.


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

Snell






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