FOUS11 KWBC 142008
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
308 PM EST Tue Jan 14 2025

Valid 00Z Wed Jan 15 2025 - 00Z Sat Jan 18 2025

...Great Lakes & Central Appalachians...
Days 1-3...

Multiple days of lake-effect snow are expected through the end of
the workweek as Arctic high pressure over the Plains advects frigid
air over the lakes, resulting in heavy snow downwind. Clipper
disturbances crossing the lakes will shift the bands north and
south, as well as introduce variability into how strong the bands
are.

The first disturbance will be exiting the lower lakes at the start
of the D1/Wednesday period this evening. Ongoing single banded
lake-effect will shift southward into the disturbance. Once the
flow becomes more northerly tonight behind the disturbance, the
single bands should shift west and break into much weaker multi-
bands as the flow becomes more perpendicular to the lakes.

This southward shift will be short-lived however as the center of
the Arctic high builds into the Ohio Valley and the next
disturbance approaches the upper lakes from the heart of Canada.
Thus, expect a rapid northward shift in the bands from west to east
as southwesterly flow builds in behind the high.

The second disturbance will be the stronger of the two as it moves
into the upper lakes Wednesday into Wednesday night. It stays
plenty cold enough for all snow. For most areas away from the lakes
the snow should remain light...however where the lakes can locally
enhance the moisture associated with this disturbance, heavy snow
is possible. For the lower lakes, the flow will remain
northwesterly into Wednesday before the northward shift occurs
Wednesday night into Thursday. With this disturbance approaching
the lake-effect should remain rather disorganized as it shifts
north.

On Thursday, the light snow with the disturbance will have
overspread all the lakes, though behind it Lake Superior will
increasingly favor the lake-enhanced areas of the northern U.P.
Further, heavier snow lifted by the central Appalachians will begin
to impact much of West Virginia and western Pennsylvania Thursday
morning. The upslope snow will continue into Thursday night before
a new high pressure area building in increases subsidence and dry
air and ends the snow.

Warm advection ahead of a third disturbance will effectively end
much, if not all of the lake-effect during the day Friday. This
third disturbance should remain almost entirely north of the
lakes, though some light snow may spread into the Arrowhead late
Thursday night.

WPC Probabilities show a low (10-30%) chance of 6 inches of snow
or more for the D1/Wednesday period southeast of the lower lakes.
There's also a low (10-30%) chance of 4 inches of snow across the
central Appalachians, particularly along the Allegheny Front of
PA/MD/WV.

...Northern High Plains...
Day 3...

An Arctic front at the leading edge of some of the coldest air of
the season will race southward out of Canada into the northern High
Plains of MT/WY/ND on Friday. Supported by the RER of the jet, a
shearing shortwave, and the contrast of warmer air ahead of the
front, this "blue norther" scenario appears highly favorable for
snow squalls, and more persistent heavy snows into the Little Belt,
Big Snowy, Bighorns, and the Absarokas.

The front will enter the Montana Hi-Line Thursday evening, racing
south down the Plains through Montana and clearing Wyoming by
Friday evening. Along with the front and favorable forcing, expect
snow squalls to develop with the front with a first peak in
intensity (based on the NAM) in the predawn hours Friday morning
across central Montana. The Snow Squall Parameter nears the top of
the scale (a 5) at this time. A secondary peak follows it during
the day Friday in Montana as the initial wave tracks down the
Wyoming Plains. The snow squalls will be characterized by rapid
changes in visibility at the initial onset of the snow as clear
conditions can drop to near whiteout in a matter of seconds. Once
the squall clears visibility improves about as fast as it initially
deteriorated. For most of the Plains, total accumulations are
expected to be minor, however since the snow will be heavy, brief
periods of hazardous driving conditions are likely. In the
aforementioned mountains, expect a more prolonged period of heavy
snow as upslope effectively wrings out the moisture, resulting in
multiple inches of accumulation.



For the Day 1-3 period, the probability of significant icing of at
least 0.10" is less than 10 percent across the CONUS.


Wegman

$$

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