Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
253 AM EST Fri Jan 2 2026

Valid 12Z Fri Jan 02 2026 - 12Z Mon Jan 05 2026

...Great Lakes...
Days 1-3...

Ongoing lake effect snow (LES) Will continue in earnest today,
especially across the eastern U.P., and downwind of Lakes Erie and
Ontario. This LES will be driven by persistent CAA across the
lakes, leading to steep lapse rates and inversion depths that climb
towards 800mb, leading to 500-750 J/kg of lake-induced instability.
While the mean low-level flow will be a bit more W than NW, this
will help tap an upstream connection from Superior, across Huron,
and towards Lake Ontario, indicating the heaviest snowfall will
likely be along and south of the Tug Hill Plateau through D1 and
into D2 before winding down in response to brief shortwave ridging.
While periods of heavy snow are also likely in portions of the
northern L.P. and along the Chautauqua ridge, the persistence and
intensity of this snowfall should be less. This is reflected by WPC
probabilities for 4+ inches on D1 that area 30-50% in the eastern
U.P. and along the Chautauqua Ridge, but above 90% east of Lake
Ontario, where locally 2+ feet is likely on D1. During D2 the
intensity and coverage of LES wanes, but WPC probabilities for 4+
inches remain above 50% near Oswego, NY.

After a brief respite in response to the aforementioned shortwave
ridging, two separate, but fast moving, shortwaves will crest the
ridge to the west and dive rapidly from Saskatchewan/Alberta across
the Great Lakes. The first of these will race southeast Saturday
aftn/night, and while some enhanced ascent and subtly increased
moisture will result in light synoptic snowfall, additional
accumulations from eastern MN through western NY should be minimal.

A more substantial impulse will then drop southeast in a similar
fashion Sunday night, but this impulse will be accompanied by
greater moisture as it originates from the Pacific and ejects from
the Northern Plains rather than central Canada. This will again be
fast moving, but will have more pronounced synoptic lift in
response to height falls, PVA, LFQ jet diffluence, and more robust
850mb WAA leading to enhanced fgen. At this time, the speed of the
system is expected to limit total snowfall, but briefly heavy snow
rates thanks to idealized ascent into the DGZ could produce a
narrow swath of more than 4 inches as reflected by WPC
probabilities that are as high as 50-70% for 4+ inches from near
Duluth, MN eastward through much of the U.P and the neighboring
portions of WI.


...Western U.S. Mountain Ranges...
Days 1-3...

An active period of weather continues across much of the West as
Pacific moisture repeatedly surges onshore in response to
persistent troughing offshore.

For D1, a brief period of ridging will blossom along the Pacific
Coast, but downstream into the Intermountain West a shortwave with
Pacific origin will lift steadily northeast while de-amplifying.
This will result in a stripe of precipitation arcing from the
Northern to the Central Rockies, and falling as snow above
6000-7000 ft. WPC probabilities D1 reach above 70% for 6+ inches
near the Tetons of WY and the Park Range of CO, with more
widespread elevated probabilities for 4+ inches across much of the
rest of the terrain in this region.

Then beginning late D1 and continuing into D3, much more pronounced
moisture begins to pivot onshore the Pacific Coast. This will be in
response to an impressive trough aligned just offshore, from which
a surface low will track northeast towards WA state by Sunday
morning, with a secondary wave tracking towards northern CA Sunday
aftn. While these surface lows will help enhance local ascent, in
general moisture will be pronounced across much of the West due to
increasing southerly flow downstream of the primary trough axis
pushing IVT well onshore with magnitudes above 500 kg/m/s. As is
typical with strong IVT plumes, they will be driven both by Pacific
jet energy and warm advection, resulting in elevated snow levels
climbing to 7000-9000 ft ahead of the first wave, and while they
will rise again with the secondary wave, they will generally be
much lower, 5000-7000 ft on D3, even lower, around 3500 ft, in the
Cascades and interior Northwest.

Any ascent forced through synoptic features or upslope enhancement
will cause rounds of heavy snowfall above these elevations both
Saturday and Sunday. WPC probabilities steadily increase and expand
through the weekend, with widespread high chances (>70%) for 4+
inches reaching from the Olympics, along the entirety of the
Cascades, the Shasta region, the Sierra, and through much of the
Northern/Central Rockies and into the Wasatch, by Monday morning.
While the heaviest snowfall is likely in the Sierra where many
locations above 6000 ft could experience 2-5 feet of snow, much of
the higher terrain of the West could receive 1-2 feet before
coverage wanes just beyond D3. This will likely bring considerable
impacts to the higher elevations due to generally low SLR snow,
with difficult travel likely across the Sierra Passes.


...Central Plains...
Day 1...

A fast moving clipper rounding the western ridge and dropping
southeast over the Central Plains today will be of subtropical
origin. Although this feature will be progressive and of modest
amplitude as it weakens, it will produce sufficient ascent into a
moistening column to produce a narrow corridor of mixed
precipitation in the vicinity of northeast Nebraska. While total
precipitation will be modest, WPC probabilities indicate a 70-90%
chance of at least 0.01" of freezing rain, with locally up to 0.1"
of ice possible leading to hazardous travel.


Weiss