198
FOUS11 KWBC 260746
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
246 AM EST Sun Jan 26 2025

Valid 12Z Sun Jan 26 2025 - 12Z Wed Jan 29 2025

...Great Lakes and Interior Northeast...
Days 1-3...

A cold and snowy period is expected through Wednesday morning
across all of the Great Lakes. A persistent longwave trough will
remain in place through the period, with several disturbances
moving through. These shortwave disturbances will locally enhance
the coverage and intensity of snowfall over and downwind (south and
east) of the lakes. Cyclonic flow will keep a steady supply of cold
air moving over the lakes, which in turn will keep the lake-effect
snows going almost constantly through Tuesday, especially east of
Lake Ontario. This morning, the first of these disturbances will
cross over the lower lakes. The associated cold front will briefly
lower temperatures and increase forcing, resulting in heavier
lake-effect behind the front through the day today. Surface ridging
will quickly build in behind the front, but the second disturbance
quickly approaches behind the ridging. This back-and-forth between
disturbance/trough and ridging will result in rapid sloshing of the
lake-effect over a larger area downwind of the lakes. However, the
bands won`t stay in any one place very long, limiting the
accumulations for most. The disturbances themselves will largely
track well north of the Canadian border, so the souther/eastern
lakes should have the more persistent lake-effect since the
associated wind direction changes will be a bit slower to occur.

A third disturbance in the form of a clipper low will move across
all of the lakes Tuesday night into Wednesday, which will simply
maintain the lake-effect while shifting the bands with the wind.
The clipper itself will make for a general light snowfall over
areas outside of the lake-effect bands as well, though where the
bands are most persistent and where topography can uplift
additional moisture (such as on the Tug Hill Plateau of NY), is
where the greatest snowfall totals are expected.

GLERL analysis of the lakes shows the average lake temperature of
around 40 degrees for Lakes Michigan, Huron, and Ontario. Lake Erie
is over 80% ice covered, and Lake Superior is around 37 degrees.
This would correspond to 850 mb temperatures needing to be at or
colder than -9 to -10 degrees Celsius in order to maintain the
minimum instability for lake-effect. Through this period, only on
Monday ahead of the second disturbance`s cold front will this
criterion not be met.

In addition to the lake-effect, the cold fronts associated with
each disturbance, especially the second one on Monday/Monday Night,
will be capable of causing snow squalls outside of the lake-effect
areas. The snow squall parameter will be over 5 at times as the
front moves over the U.P. late Monday afternoon, and the St.
Lawrence Valley Monday night. The parameter will lower a bit to
between 2 and 4 as it moves over the rest of New York State and New
England through Tuesday morning.

WPC probabilities for over 8 inches of snow through D3/Wednesday
morning are high (>80%) across the Tug Hill Plateau and western
Adirondacks of northern New York. Probabilities are moderate
(50-70%) across the Lake Superior shoreline of the eastern U.P.,
and low (10-30%) for northern Michigan and far western New York
south of Buffalo, due to Lake Erie being largely frozen over.

...Southwest, Great Basin, and Central Rockies...
Days 1-3...

An upper level cutoff low will begin the period this morning over
central California. This feature will be the focus for wintry
weather across the mountains of the Southwest through Wednesday
morning. The upper level low will drift south into southern
California through Monday, then drift eastward into Arizona through
Wednesday morning. Due to the slow movement of the low and the cold
air associated therewith, snow will be the dominant precipitation
type over many of the higher elevations. The low will also bring
much needed rainfall to the fire stricken areas in and around Los
Angeles.

While the cutoff low itself will be slow-moving, plentiful upper
level energy/shortwaves rotating around the low will locally
increase precipitation intensity, especially where flow off of the
Pacific lines up orthogonal to the terrain. This is most likely in
the southern Sierras and portions of the Transverse Ranges,
especially northeast/inland of Los Angeles. WPC probabilities of 8+
inches of snow are low (10-40%) for the southern Sierras and
portions of the Transverse Ranges through Wednesday.

By Tuesday, the cutoff low moving into Arizona will shift the
greatest lift and divergence east into the Four Corners region. A
lack of moisture generally over this area should confine the areas
of potential heavy snow to the higher elevations of Utah and
Colorado, but amounts are generally unlikely to exceed 8 inches.


For the Days 1-3 period, the probability of significant icing
greater than 0.10" is less than 10 percent.

Wegman



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