Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
137 PM EST Fri Jan 24 2025

Valid 00Z Sat Jan 25 2025 - 00Z Tue Jan 28 2025

...Great Lakes...
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. There may be
as many as 4 weak shortwaves through the period, 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.

The exception will most likely be east of Lake Ontario and across
the Keweenaw Peninsula, where more persistent westerly fetch will
promote continued LES. For areas east of Lake Ontario (Tug Hill and
western Adirondacks) a heavier burst is likely on Sunday as a
potent short wave crosses over the region with uniform westerly
flow through the column and near the LFQ of a departing upper jet.
This will promote a single LES band with the potential of
containing 1-2"+ /hr rates. This is reflected by WPC probabilities
for 8+ inches exceeding 70% both D2 and D3 east of Lake Ontario and
into the Tug Hill Plateau where 3-day snowfall of 1-2+ feet is
possible. 4+ inch probabilities exceeding 90% also exist across
the Keweenaw on D1 where locally as much as 12 inches is possible.


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

A shortwave digging south and amplifying as it exits the Pacific
Northwest tonight will continue to strengthen and eventually close
off over central CA late on Saturday. 500-700mb height falls are
forecast to dip as low as the 1st percentile according to the NAEFS
climatology. This feature will likely then crawl southward as it
remains cutoff, with multiple closed height contours, over CA
through D3, reaching potentially as far south as the Los Angeles
area by the end of the forecast period. This amplified closed low
development and the accompanying longwave trough will force
downstream jet development, as the subtropical jet arcs northeast
from near Baja into the Central Plains, reaching as high as 110 kts
D2, and then as high as 150kts D3 as secondary enhancement occurs
over CA.

This evolution will have a two-pronged effect on the precipitation
and snowfall across the West. First, the shortwave digging south
will push a cold front southward beneath it, causing snow levels to
crash rapidly in its wake from 3000-5000 ft to below 500 ft,
although across the southern Great Basin and southern CA snowfall
levels will fall only to around 3000 ft. Most of the precipitation
D1-D2 associated with this front will be modest due to normal, to
below normal, PWs. However, the developing jet streak combined with
the frontal passage and post-frontal upslope flow will result in an
axis of stronger ascent through fgen (and the upslope), leading to
a swath of heavy snowfall from the Absarokas of MT southward
through WY, and most impressively into the Colorado Rockies,
including the Park Range, D1-2. The strongest fgen will likely
reside west-to-east from CO through the Sierra, providing
additional heavy snow accumulations for portions of UT and NV. WPC
probabilities D1-2 are moderate to high (50-90%) for 4+ inches
across these areas, with the most substantial snowfall likely
across CO where 12-18 of total snowfall is possible.

D2-D3 snowfall begins to ramp up downstream of the slowly sinking
closed low, in response to increasing WAA/moist advection and
impressive deep layer ascent as mid-level divergence overlaps with
increasing LFQ jet-level diffluence. This will spread periods of
moderate to heavy precipitation northeast into the Sierra, as well
as the Transverse and Peninsular Ranges of CA, with some moisture
spilling into the Great Basin (including Mt. Charleston) as well.
The airmass across this region is likely to be characteristically
different from that farther to the north as the front stalls, so
snowfall in this area will be more elevated and with lower SLR.
Still, the favorable ascent and moisture overlap will likely
produce rounds of heavy snow, and WPC probabilities D2 are
moderate (50-70%) for 4+ inches in the Sierra, and expand D3 into
the southern CA ranges, reaching 70-90% for 4+ inches, and as high
as 20-50% for 8+ inches. The highest accumulations expected above
5000 ft, but snow levels could dip as low as 3500 ft underneath the
upper low.


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


Weiss/Snell