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

Valid 00Z Mon Jan 27 2025 - 00Z Thu Jan 30 2025

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

A cold and snowy period is expected through Wednesday across the
Great Lakes and Interior Northeast as cyclonic flow and several
disturbances continue to influence a favorable pattern for bouts of
light-to-moderate snowfall. These shortwave disturbances will
locally enhance the coverage and intensity of snowfall over and
downwind (south and east) of the lakes. The Great Lakes have cooled
significantly over the last few weeks, with overall ice coverage
around 24% and Lake Erie alone up to 85%. The warmest lake
temperatures remain around 5 degrees C across lakes Ontario, Huron,
and Michigan, which is still favorable for LES as 850 mb temps drop
to as low as -20 degrees C. For the D1 period (00z Mon- 00z Tues)
brief ridging between systems will lower LES coverage due to more
southwesterly flow as a potent arctic cold front approaches the
Upper Great Lakes Monday evening. As this front crosses the region
and enters the Northeast Tuesday morning, a period of lighter
snowfall ahead of the front is likely across and just downwind of
the Great Lakes associated weak WAA, and with somewhat lower SLRs
given the very strong low- to- mid level flow fracturing dendrites.
Of somewhat more concern with the arctic boundary will be
associated with gusty winds and the potential for snow squalls as
the front progresses south and east. CAMs and other global guidance
continue to highlight elevated snow squall parameters as strong
fgen and relatively steep low-mid-level lapse rates promote a quick
burst of snow along the front through early Tuesday in the Interior
Northeast and New England. The greatest potential exists across
the Upper Great Lakes Monday evening and early Tuesday morning
across Upstate New York. These values and the snow squall threat
greatly decrease by mid- morning as the front loses it's punch once
it reaches the northern Mid- Atlantic.

A second disturbance in the form of a clipper low will move across
the Great Lakes Tuesday night into Wednesday, beginning in the
Upper Lakes and sliding eastward to New England on Wednesday. This
will maintain the lake- effect/enhanced snow 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.

WPC probabilities for over 8 inches of snow on D2-D3 are highest
(50-70%) across the Tug Hill Plateau and western Adirondacks of
northern New York. Probabilities for at least 4 inches of snow are
medium (40-70%) across the Arrowhead of MN, U.P. and northern L.P.
of Michigan and high (>80%) across far western NY.

...Southwest and Central/Southern Rockies...
Days 1-3...

An upper level cutoff low will continue to impact the southern
Sierra and southern CA ranges through D1 until the system slides
inland and moves very slowly across the Southwest D2-D3. Due to
the slow movement of the low and the cold air aloft associated
therewith, snow will be the dominant precipitation type over many
of the higher elevations with snow levels D1 across southern CA
around and slipping just under 4000 ft.

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. A strong, but weakening
upper southwesterly oriented jet off the Pacific will also allow
for increased diffluence just to the north across the Southwest
through midweek. D1-D2 WPC probabilities for at least 4 inches are
high (>70%) across the southern Sierra, southern CA ranges, and
into the higher terrain of the Southwest, including parts of the
Mogollon Rim of AZ.

By Wednesday, the cutoff low moving into Arizona will meander east
with the greatest lift and divergence situated into the Four
Corners region and southern Rockies. A lack of moisture generally
over this area to begin with should confine the areas of
potentially heavy snow to the higher elevations of southern Utah
and Colorado, but by late D3 a resurging upper level jet streak
developing at the base of the upper low will strengthen and bring
another round of upper level divergence with enhanced moisture
advection entering the southern Plains (>90th climatological
percentile per the ECMWF). This leads to medium chances (40-60%)
for at least 4 inches of snowfall on D3 across the higher
elevations of the Four Corners, Mogollon Rim of AZ, and southern
Rockies of CO/NM. Greater coverage of snowfall is expected just
after the end of the D3 forecast period (00z Thurs) throughout the
southern Rockies. Snow levels are currently forecast to range from
around 4000 ft under the upper low over AZ and up to 5300 ft
across the Southern Rockies on Wednesday.


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

Snell