205
FOUS11 KWBC 211901
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
201 PM EST Thu Nov 21 2024

Valid 00Z Fri Nov 22 2024 - 00Z Mon Nov 25 2024

...Pacific Coast through the Interior Northwest and Great Basin...
Days 1-3...

Persistent closed low centered off the Pacific Northwest coast will
become re-energized by vorticity maxima swinging through the
associated trough this period. This will result in amplification
of the gyre, noted by an expansive region of 500-700mb height
anomalies reaching below -1 sigma, and approaching -3 sigma in the
core of the trough. Beneath this trough, deepening surface low
pressure will approach the WA coast Saturday, but likely get pulled
back to the west before landfall in response to secondary
shortwave energy digging to its south. This will result in
persistent onshore and divergent mid-level flow, accompanying
upper level jet energy to spread moisture onshore as a continuation
of the ongoing atmospheric river (AR) through Saturday. While
probabilities for IVT exceeding 500 kg/m/s wane and pivot southward
through D2, this will still result in plentiful moisture and
widespread moderate to at times heavy precipitation pushing onshore
the Pacific Coast into the interior Northwest.

Snow levels within this AR will be quite high, generally 6000-8000
ft, except east of the WA Cascades on D1 when they will remain
entrenched around 4000 ft before rising. This indicates that most
of the heavy snow will be above pass levels, except in the higher
Cascade Passes D1, and the Sierra Passes D2. WPC probabilities D1
are high for more than 6 inches of snow in the WA Cascades, the
Salmon River Range, and portions of the Northern Rockies near
Glacier NP. On D2, snowfall is more robust and expansive, with a
greater than 90% chance of 6 inches again in the Northern Rockies
and Salmon River/Sawtooth Ranges. More impressively, the Sierra
will be favorably located to be impinged upon by moisture D2,
leading to pronounced upslope snow. WPC probabilities are above 80%
for 12+ inches on D2, with 2 feet possible above 8000 ft.

By D3, snow levels crash again as a cold front pushes onshore, but
this is accompanied by reduced moisture advection as the column
begins to dry out. Some moderate snow will persist through onshore
flow, especially in the Sierra where an additional 6-12 inches is
possible, but otherwise WPC probabilities for more than 6 inches
are between 30-50% and focused across the Wasatch, Uintas, and
ranges of NW WY.


...Great Lakes, Appalachians, Northeast...
Days 1-3...

Impressive upper low over the Great Lakes will feature a complex
evolution through the weekend resulting in multiple surface low
pressure and varying areas of heavy snow.

The first surface low will track south across the Ohio Valley and
towards the Central Appalachians tonight into Friday. Immediately
downstream of this feature, a band of moderate to at times heavy
snow is likely, but the fast progression and marginal thermal
structure of the antecedent column will limit overall
accumulations.

The exception to this will be across the Central Appalachians
especially from the Laurel Highlands southward across WV and as far
south as the Great Smokey Mtns of TN where prolonged upslope flow
ahead of this wave, combined with the wave itself, and then
secondary upslope flow on Friday night will result in waves of
heavy snowfall. WPC probabilities in this area are high (>70%) on
D1 for more than 8 inches of snow, with probabilities for an
additional 4+ inches on D2 reaching 30-50%. Event total snowfall
could peak around 2 feet in central WV. Otherwise, snowfall at
lower elevations and into the Ohio Valley should be 1-3" or less.

A second, and likely stronger area of low pressure developing south
of New England today will retrograde back to the NW while deepening
as it remains embedded within the large 500mb gyre overhead. As
this occurs, an impressive plume of low-level moisture will wrap
northwest into the system, driving rich theta-e advection into a
TROWAL to support some elevated instability and enhance available
moisture for heavy precipitation. During this evolution,
deformation will increase on the W/SW side of the system as it
becomes exceptionally wrapped, with CAA on the southern side
leading to improved fgen response beneath this deformation axis.
This setup will support intense mesoscale ascent and bands of heavy
precipitation rotating W/SW as the low retrogrades. Although the
column will be marginally supportive of accumulating snow outside
of higher elevations (above about 1500 ft), the intense lift should
dynamically cool the column and allow efficient snowfall
accumulation at times even in lower terrain. SLRs for this event
will likely be below climo due to near freezing temps and warm
soils/compaction, but still, rates of 1-2"/hr will promote heavy
accumulations, especially in the Poconos and Catskills and
surrounding areas. WPC probabilities D1 are high (>70%) for 8
inches, with 2-day snowfall potentially reaching above 12" in a few
spots. This will likely result in dangerous travel and impacts to
infrastructure including scattered power outages.

In the lower elevations of the Mid-Atlantic, WPC probabilities are
high for more than 2 inches as far west as near BUffalo, NY and
central PA, but locally higher totals are likely where these bands
pivot. Additionally, the guidance has become more aggressive
pinwheeling some snow bands as far SE as the I-95 corridor, so
while accumulations in the major urban areas from Washington, D.C.
to New York City are expected to be minimal, if any, these cities
could see their first "falling" snow of the season Friday
morning/aftn.

Along the Adirondacks, Green, and White Mountains, the
upper low will draw moist easterly flow northward and into these
ranges Friday night through Saturday while yet a third surface low
develops east of Maine and pivots northwest. Snow levels should be
high enough that only the highest terrain will receive significant
snow, but WPC probabilities have increased and now feature a 30-50%
chance on D2 and D3 of more than 4 inches, primarily in the White
Mountains of NH and the peaks of northern ME.


...Northern Rockies through the Northern Plains...
Days 2-3...

A shortwave ejecting through the base of an anomalous trough off
the Pacific Coast will swing eastward into the Northwest Saturday
and then begin to amplify into a potent but fast moving impulse
across the Northern Rockies and into the Northern Plains Sunday.
This deepening system will drive downstream ascent through height
falls, PVA, and divergence. At the same time, a zonally oriented
jet streak will pivot south of the region, placing its favorable
LFQ atop the High Plains, interacting with increasing baroclinicity
as a front wavers near the international border. The overlap of
synoptic lift with the enhanced mesoscale ascent through fgen will
result in weak cyclogenesis, leading to an expanding area of
precipitation falling as snow. The column will be plenty cold for
wintry precipitation thanks to high pressure from Canada extending
southward, and as the WAA along the warm front lifts north, it will
result in an exceptionally deep DGZ to support fluffy aggregate
dendrites which should accumulate efficiently.

The heaviest snowfall is likely  in the terrain near the Northern
Rockies where upslope flow will enhance ascent, and WPC
probabilities support a high risk (>70%) for at least 8 inches near
Glacier NP both D2 and D3. However, heavy snowfall is also likely
farther east across northern MT and into ND where the most robust
synoptic ascent will materialize. The guidance has trended a bit
farther north today, but current WPC probabilities suggest around a
30% chance for more than 4 inches along the international border as
far east as central ND through D3.


The probability of significant ice across the CONUS is less than
10 percent.

Weiss



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

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

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