FOUS11 KWBC 100759
QPFHSD

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

Valid 12Z Tue Dec 10 2024 - 12Z Fri Dec 13 2024

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

Broad SW flow in the mid-levels to the south/southeast of a
shortwave nearing James Bay will override a nose of high pressure
from Atlantic Canada, maintaining sub-freezing low-levels across
much of central/northern New England this morning. Wavy frontal
boundary will try to lift northward into the colder air, but will
be slow to succeed until tomorrow (Wed). As precipitation moves
into the region overnight tonight into Wednesday morning, freezing
rain will be likely over much of central/northern New England,
especially in areas with a little elevation. On Wednesday,
southerly/southeasterly flow will eventually win out at the low
levels, turning all areas over to plain rain. Ice accumulations of
a tenth or two of an inch are likely over parts of the Green and
White Mountains and into western Maine where the cold air may hold
on the longest. WPC probabilities for at least a tenth of an inch
of ice are >50%. Though the probabilities of at least 0.25" are
low, this may be underdone as the models are sometimes too quick to
have the cold air retreat.


...Central Appalachians... Day 2...

The southern part of the system affecting the Northeast tonight
into Wednesday will be the evolution of an increasingly negatively-
tilted and deep upper trough over the eastern 1/3 of the CONUS.
Frontal boundary will clear past the Appalachians tonight, allowing
colder air to push in behind it and change rain to snow over the
Cumberland Plateau and points eastward. Upslope enhancement into
the central Appalachians will promote more modest snow totals, and
WPC probabilities for at least 4 inches of snow are >30% over
eastern WV northward through the western MD Panhandle (Garrett Co)
into the Laurel Highlands in PA.


...Northern Plains... Day 1...

The leading edge of the encroaching Arctic air-mass will track
into the Northern Plains today. Surface-based warming out in front
of the Arctic front and strong CAA aloft will allow for lapse rates
in the 500-700mb layer to be >7.5C/km in some cases, with just
enough low-level moisture for some snow squalls to develop. These
squalls could form as far north and west as eastern Montana and
western North Dakota this morning then race southward and eastward
into South Dakota this afternoon and the Missouri Valley by this
evening. Squalls could even continue to track into parts of Iowa,
northern Kansas, and northern Missouri tonight as the snow squall
parameter exceeds 1 in much of the CAM and even global guidance.
Though snow amounts will be light, squalls can lead to rapid
reductions in visibility with accumulating snow on roadways where
road temperatures drop below freezing and create icy hazards.


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

Upper pattern will become increasingly amplified over the East as
ridging into the West weakens but builds quite smartly over the
northern Atlantic. This will help strengthen the jet to ~170kts out
of the southwest across the TN/OH Valley overnight tonight as
arctic air moves in from the northwest. Lake effect snow will pick
up in earnest starting this evening and overnight across the U.P.
of Michigan and then into the western side of Lower Michigan as
height falls rush through (500mb heights below the 1st percentile
per the CFSR) as the trough axis becomes quite negatively-titled
into the Southeast/Mid-Atlantic. 850mb temperatures will plunge to
-15C to -25C tomorrow with plenty of Lake-to-850 DeltaT. Snow will
 continue through the period as winds slowly back from NW to WNW or
 W as the upper center passes by. Into the Northeast, once the
 front clears the area and temperatures fall below freezing area-
 wide, snow will increase off Lakes Erie/Ontario, the latter of
 which will support a strong single band into the Tug Hill Plateau
 with lighter snow pretty far inland. Snow off Lake Erie may also
 be a single banded aimed just into the BUF southtowns. Snow will
 continue across all the Great Lakes through the end of this period
 (12Z Fri) but will continue beyond then.

Through 12Z Fri, WPC probabilities of at least 12 inches of snow are
high (>70%) across the favored lake belts on NW flow over the U.P.
and northwest Michigan; between Erie, PA and Buffalo, NY (esp the
southtowns but perhaps into the southern Buffalo area); and in the
Tug Hill. Snowfall rates of 1-2"/hr (surpassing 3"/hr at times) are
likely in the more intense bands.


...WA/OR Cascades... Day 3...

Incoming shortwave or upper low will move into NorCal/Southwestern
OR Thursday, spreading some precipitation into the Cascades
southward into the Sierra. Snow levels will be around 3000ft to the
north and 5000ft to the south with QPF totals generally up to
0.50" or so. WPC probabilities for at least 6 inches of snow are
highest in the WA/OR Cascades.

Fracasso

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