FOUS11 KWBC 240819
QPFHSD

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

Valid 12Z Fri Jan 24 2025 - 12Z Mon Jan 27 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. This is reflected by WPC probabilities for
4+ 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, and by 4+ inch probabilities exceeding 90% across the
Keweenaw on D2 where locally as much as 12 inches is possible.


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

A shortwave digging south from British Columbia will begin to
amplify as its accompanying vorticity surges into WA/OR this aftn.
This feature will continue to dive southward while amplifying,
reaching CA by the start of D2, and most guidance now supports the
development of a closed low over central CA Saturday aftn/evening
where 500-700mb heights fall as low as the 1st percentile according
to the NAEFS climatology. This feature will likely then crawl
southward as it becomes cutoff, with multiple closed height
contours, over CA through D3, reaching potentially only 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 10-30% for 8+ inches, with the highest accumulations expected
above 5000 ft.


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


Weiss

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