Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
241 AM EST Sat Jan 25 2025

Valid 12Z Sat Jan 25 2025 - 12Z Tue Jan 28 2025

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


Cyclonic flow will persist across the Great Lakes and Northeast
through the period, although modest amplitude of this troughing
will result in generally progressive flow. Within this flow several
shortwaves will traverse WNW to ESE atop the region, each one
driving renewed surges of CAA across the Great Lakes. Two
additional shortwaves are progged to move across the region, one
Saturday night, and another Sunday night, with subtle thickness
rises in between each feature, and this will result in waves of
lake effect snow (LES) with variable wind direction driven by CAA
behind each impulse. The Great Lakes have cooled dramatically in
the past 7-10 days, reflected by the warmest waters now generally
around +5 C, and regional ice coverage up to 24% (85% on Lake Erie
now), which will somewhat limit the intensity of any LES, and the
heavy snow during this period will be more driven by repeated
rounds of moderate snow than very heavy rates, but favorable fetch
across Lake Superior D1, and Ontario D2 will produce rounds of
heavy LES with rates 1"/hr or more possible. This is reflected by
WPC probabilities that are moderate (50-70%) for 6+ inches across
the Keweenaw Peninsula D1, and high (>90%) for 6+ inches D2 into
the Tug Hill Plateau.

Then on D3, the core of the cyclonic gyre begins to drop south
from the Hudson Bay, driving sharp height falls and extremely
confluent mid-level flow to its south. As this dives southward, it
will push a cold front draped west to east from Saskatchewan to
Upstate NY by the end of D3, causing enhanced ascent and strong
flow across the Lakes. This will result in additional moderate to
heavy snow, with expansion into Upstate NY, especially in the
upslope region of the Adirondacks, Monday. While moisture is
expected to be significant across due to both synoptic and lake
enhancement, there is some uncertainty as to how the snowfall will
accumulate due to SLRs that may be extremely fractured due to
strong winds within an otherwise favorable DGZ. The models have
trended upward with snowfall, however, and current WPC
probabilities for more than 4 inches of snow are as high as 70-90%
downstream of Lake Ontario and into the Adirondacks.


...The West...
Days 1-3...

A strung out lobe of vorticity emanating from a shortwave closing
off over Northern California this morning will help push a cold
front southward, but with only a lazy loss of latitude through the
period. This slow evolution will be in response to the
amplification of the mid-level pattern, as the shortwave deepens
into a closed low and then drifts over central CA through at least
Sunday night before finally dropping farther south towards northern
Baja late Monday /D3/. Height falls, although slow, will be
impressive as 500-700mb heights drop to around the 1st percentile
according to the NAEFS climatology, helping to drive deep layer
ascent through the region. This ascent will be additionally
enhanced by two distinct downstream jet streaks, one lifting away
into the Central Plains Sunday, with a more intense jet streak
developing over southern CA/northern Baja Monday. The overlap of
the LFQ of this jet streak, the mid-level height falls, and
impressive downstream mid-level divergence will result in
increasing lift across the Desert SW and Great Basin, leading to
expanding rounds of precipitation Sunday and Monday before
weakening and drying out late in the period. This will be in
addition to a round of heavy snow along the front and enhanced by
isentropic ascent, fgen, and upslope flow from the Sierra east to
the CO Rockies on D1.

WPC probabilities for heavy snow D1 are aligned west to east just
north of the surface front, from the central Sierra through the
northern CO Rockies, where they reach 70-90% for 6+ inches, highest
in the Park Range of CO, the Wasatch Front of UT, and the Sierra
in CA where locally as much as 10" of snow is possible on D1.
Although amounts are less otherwise, a fairly continues stripe of
2-4" of snow is possible along this axis today and tonight.

As the upper low consolidates, moisture and ascent become focused
in the southern half of CA, the Great Basin, reaching the Four
Corners by D3. Snow levels will hover around 3000-4000 ft during
this time, resulting in heavy snow in much of the higher terrain as
far south as the San Bernardino and San Jacinto Mountains. WPC
probabilities D2 are moderate (30-50%) for 6+ inches in portions of
the southern Sierra, as well as most of the Peninsular and
Transverse ranges, and also for Mt. Charleston in NV. By D3,
precipitation expands east, and WPC probabilities for 4+ inches
reach 10-30% across the Kaibab Plateau of AZ, with additional
significant snowfall again progged over Mt. Charleston and the San
Bernardino Mountains.


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


Weiss