FOUS11 KWBC 220831
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
331 AM EST Fri Nov 22 2024

Valid 12Z Fri Nov 22 2024 - 12Z 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
remain 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 today, but likely get pulled 
back to the west on Saturday 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) across northern CA through 
early Saturday. While IVT values exceeding 500 kg/m/s wane and 
pivot southward by the end of D1, 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 where 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 D1-D2. WPC probabilities 
D1 are high for more than 6 inches of snow in the northern 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 by the end of D1, leading to pronounced upslope 
snow. WPC probabilities are above 80% for 12+ inches through
Saturday, with 2 feet possible above 8000 ft.

By D2 onward, 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.

...Appalachians and Northeast... Days 1-2...

Impressive upper low over the Northeast will feature a complex 
evolution through the weekend resulting in a couple surface lows and
varying areas of heavy snow.

The Central Appalachians, especially from the Laurel Highlands 
southward across WV and as far south as the Great Smokey Mtns of
TN/NC, will benefit from prolonged upslope flow on the western 
periphery of the upper low resulting in waves of heavy snowfall
through Saturday. WPC's snowband probability tracker depicts an
extended period of 1-2"/hr snowfall rates during the D1 period
across the Allegheny Mtns. WPC probabilities in this area are high
(>70%) on D1 for more than 8 inches of snow. Event total snowfall 
could peak around 2 feet in central WV.

The ongoing potent and compact surface low churning near Long
Island is forecast to loop westward and eventually toward the south
today over eastern PA as it remains embedded within the large 
500mb gyre overhead. As this occurs, an impressive plume of low- 
level moisture will continue to 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 early on D1. Increasing deformation is noted on
current radar trends on the W/SW side of the system as it becomes
exceptionally wrapped. 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 early this morning. Most of the 
ongoing snowfall is expected to occur before or right around the 
start of the D1 period (12z Fri), so WPC probabilities don't 
reflect the ongoing event. Nonetheless, the greatest snowfall 
amounts are expected in northeast PA and neighboring areas of New
York and northern NJ, with 2-day snowfall potentially reaching 
above 12" in a few highly elevated spots. This will likely result 
in dangerous travel and impacts to infrastructure including 
scattered power outages.

In the lower elevations of the Mid-Atlantic, some snow bands could
pivot 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 another 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 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 1-2...

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 far northern 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
biggest question remains whether or not this favorable environment
and most impactful snowfall will span south over the U.S.-Canada border.

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 on D2. However, heavy snowfall is also possible farther
east across northern MT along the international border D1-D2 where
the most robust synoptic ascent will materialize. A leading surge
of modest upper diffluence and WAA may lead to a burst of snow
today across northwest and northern MT. WPC probabilities for at
least 2" on D1 are high throughout northern MT. For the main 
activity related to the approaching shortwave on D2, the guidance 
has continued to trend a bit farther north from central MT eastward
to ND, with current WPC probabilities suggest around a 30% chance 
for more than 4 inches right along the international border as far 
east as north- central MT on D2, with higher probabilities for
heavy snowfall located in southern Alberta and Saskatchewan.

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

Snell

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

https://www.wpc.ncep.noaa.gov/key_messages/La...

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