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

Valid 00Z Sun Jan 26 2025 - 00Z Wed Jan 29 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 this
weekend, one tonight and another Sunday night, with subtle
thickness rises in between each feature. 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 25% (80% on
Lake Erie), which will somewhat limit the intensity of any LES. The
heavy snow during this period will be more driven by repeated
rounds of moderate snow rather than very heavy rates, with the
setup for more favorable fetch across Lake Ontario D1 into early
D2. This is reflected by WPC probabilities that are moderate
(50-70%) for 8+ inches 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
northern New England early Tuesday, 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, beginning Monday night.
While moisture is expected to be significant (IVT above the 97.5th
climatological percentile per the 00z NAEFS) 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.
Additionally, as the cold front crosses the Northeast and more
specifically New England Tuesday morning, low to mid-level lapse
rates increasing to around 5-7 degrees C/km could prompt snow
squalls making eastward across the elevated terrain. Most guidance
highlights elevated snow squall parameters extending from NY and
northern PA through New England during the first half of Tuesday.
For snowfall, current WPC probabilities for at least 6 inches of
snow are as high as 70-90% downstream of Lake Ontario and into the
Adirondacks.

Additionally, a trailing clipper system on the heels of this arctic
front is forecast to slide into the Upper Great Lakes by the end of
Day 3, with favorable mid-level fgen ahead of an associated surface
low. WPC probabilities for at least 4 inches of snow ending 00z
1/29 are currently low (10-30%) from the Minnesota Arrowhead to
northern Michigan.


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

A potent upper trough extending from northern CA to the northern
Rockies will see the bottom lobe of vorticity completely close off
tonight and help push a cold front southward across the Southwest.
This upper low will then gradually lose latitude before crossing
southern CA D2 and entering the Southwest D3. This slow evolution
will be in response to the amplification of the mid- level pattern,
as the closed low meander underneath the building upper ridge
arching over top into the Pacific Northwest and British Columbia.
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 early on D1.

WPC probabilities for heavy snow D1 are confined to the Sierra
Nevada and Park Range of CO as snowfall weakens across the
Intermountain West tonight, with low chances (10-30%) across the CO
Rockies and high chances (70-90%) in the central/southern Sierra.
Amounts in the Sierra could approach 1 foot by the end of D1.

As the upper low consolidates, moisture and ascent become focused
in the southern half of CA, the Great Basin, reaching the Four
Corners through 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, and higher (50-70%) across the
San Gabriel Mts and northern San Jacinto Mts, 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.


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


Weiss/Snell