FOUS11 KWBC 110852
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
352 AM EST Tue Feb 11 2025

Valid 12Z Tue Feb 11 2025 - 12Z Fri Feb 14 2025


... Part 2/2 ....

...Sierra Nevada into much of the West... Days 1-3...

As a large trough amplifies across the Intermountain West,
upstream shortwaves will begin to dig along the Pacific coast as
vorticity impulses shed east from an amplifying closed low over the
Pacific. While this low won't really become organized until late
D3, energy pivoting onshore beginning late D1 will spread an axis
of moisture into central and southern CA, with snow in the terrain
above generally 3000-4000 ft. This impulse is weak and will be
directed almost due SW by the downstream trough, but WPC
probabilities are moderate (30-50%) for more than 6 inches across
primarily the southern Sierra D1.5.

After this first impulse, the closed low offshore begins to deepen
and pivot eastward. Height anomalies drop steadily offshore,
reaching as low as 1st percentile within the CFSR climatology by
00Z Friday near the CA/OR border, and the accompanying and
impressive downstream divergence will begin to displace the trough
with a brief period of shortwave ridging. Increasing moisture
advection onshore, driven by confluent mid-level flow south of this
closed low topped by a collocated strengthening jet streak will
surge IVT as high as 500 kg/m/s into the coast late Thursday. This
will manifest as expanding and intensifying precipitation, with the accompanying warm front/WAA lifting snow levels rapidly to as much
as 6000 ft in the Sierra. However, farther north into the
Shasta/Trinity region and into Oregon, pronounced cold air will be
slow to retreat, leading to some lowland snow potential as well as overrunning/freezing rain. Moisture will then quickly spread
throughout the Intermountain West through the end of the period
(12z/Fri) along with the progressing, but weakening upper trough.
There continues to be some uncertainty in timing and placement,
but there is high confidence that this event will eventually result
in heavy wintry precipitation across the West. Current WPC
probabilities are high (>90%) for 18+ inches across much of the
Sierra, generally above 4000 ft, and above 70% for 12+ inches (above
3000 ft) in the Shasta/Trinity/Klamath region. This could be quite
impactful to many area passes. Further east into the Intermountain
West and central Rockies, high probabilities (>70%) for at least 8+
inches of snow exists across the Wasatch into the Tushar Mts of
Utah, as well as the San Juan Mts of Colorado.

...Northeast... Days 1 & 3...

Today, a strong cold front will race from the Great Lakes through
New England, bringing with it an increased risk for convective snow
showers and snow squalls. The high-res CAMs remain aggressive with
their depiction of simulated reflectivity along this front Tuesday
aftn, suggesting a greater threat for snow squalls. The greatest
risk appears to be from northern Upstate NY across northern New
England which is where the best overlap of 0-2km fgen and RH
overlap, and although instability is modest, the SnSq parameter
does reach +2 across this region on the 00z GFS. This may end up
more as convective snow showers than true squalls, but briefly
intense snow rates and restricted visibility could cause dangerous
travel this afternoon/evening.

Then during the end of D2 and into D3 (centered around 00Z Thursday
to 00z Friday) more widespread significant precipitation will
overspread the region from SW to NE. This precipitation will be
associated with a robust low pressure lifting across the Ohio
Valley, driven by a modest shortwave on the downstream edge of a
deepening trough over the middle of the CONUS, overlapped with the
RRQ of a 150 kt poleward arcing jet streak moving across the
eastern Great Lakes. Downstream of this system, moist advection
will maximize in response to low- level WAA on S/SE flow
overrunning a retreating high pressure that will try to wedge back
to the west into New England. This will result in an expansion of
wintry precipitation, likely starting as snow everywhere before
gradually transitioning to sleet and freezing rain, especially
south of I-90, before winding down Thursday night.

There is growing confidence with the speed of the system, with at
least moderate snowfall accumulations likely, especially in higher
terrain and northern Maine, as reflected by WPC probabilities that
are above 50% for 4+ inches from the Adirondacks across much of
northern New England, with >80% probs in northern Maine.
Additionally, light to moderate icing is possible as reflected by
WPC probabilities of 10-30% for 0.1" in the Catskills, Berkshires,
and Litchfield Hills.

Weiss/Fracasso/Snell

...Winter Storm Key Messages are in effect. Please see current
Key Messages below...

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png

https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_2.png

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