FOUS11 KWBC 150737
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
337 AM EDT Tue Oct 15 2024

Valid 12Z Tue Oct 15 2024 - 12Z Fri Oct 18 2024

...Northeast...
Day 1...

Following a brief tapering off of snowfall overnight, periods of
snow will pick back up over the northern Appalachians Tuesday and
linger into Tuesday night. This is due to an elongated upper level
trough pivoting around the western flank of a closed 500mb low
situated over the St. Lawrence Seaway that will traverse the
Northeast from west to east. In addition to the usual upslope flow
into the Adirondacks, Green, and White Mountains, the steep lapse
rates aloft will trigger potent snow showers that could cause brief
bursts of heavy snow at elevations >2,000ft. A few snow showers may
linger around northern New England Wednesday morning, but
accumulations would be minor and limited to the elevations >3,000ft
on Wednesday. WPC probabilities show low chances (10-30%) for
additional snowfall totals >4" along the higher elevations of the
Adirondacks, Green, and White Mountains through Wednesday morning.

...Pacific Northwest...
Days 2-3...

An upper level shortwave trough bringing a surge in 850-700mb
moisture and causing 500-700mb heights falls will cause snow levels
to gradually drop from 6,000ft Tuesday night to around 4,000ft
Wednesday evening over Washington's Olympic and Cascade Ranges. As
this initial 500mb vort max exits to the east Wednesday night, a
second trough will dive south through the region on Thursday with
snow levels bottoming out just below 4,000ft in some cases. Periods
of heavy snow are expected along the Oregon Cascades during the day
on Thursday. Snow should taped off some time Thursday night. WPC
PWPF showed high chances (50-70%) for snowfall totals >4" above
5,000ft in the Cascade Range through Thursday.

...Northern & Central Rockies...
Day 3...

Pacific moisture out ahead of the first upper level shortwave to
traverse the Pacific Northwest is forecast to produce periods of
snow in the higher elevations (>6,000ft in northern ID, >7,000ft in
the Bitterroots, Sawtooth, Absaroka, Tetons, Wind River, and Big
Horn Ranges) early in the day on Thursday, but it is with the
approach of the second trough where snowfall rates look to
increase. Stronger height falls Thursday evening will accompany an
air-mass that NAEFS shows at the 500mb and 700mb layers to be below
the 10th climatological percentile. Plus, NAEFS shows an anomalous
IVT that tops the 90th climatological percentile from the Great
Basin to Utah. The region will also be placed favorably beneath a
coupled jet structure that will help to maximize upper level
divergence from the Absaroka on south to the Wasatch Thursday night
and into Friday morning. Waves of low pressure will develop along
a strengthening frontal boundary that will track through the
Wasatch and into southern WY by Friday morning, prompting
precipitation that starts out as rain to make a quick changeover to
heavy snow.

WPC PWPF sports moderate-to-high chances (50-70%) for >6" of
snowfall in the Absaroka of southwest Montana with some moderate
chances (40-60%) for >12" in some of the Absaroka's tallest peaks.
Farther south, the Bighorns and Wind River Ranges sport low-
moderate chances (30-50%) of seeing >8" of snowfall through 12Z
Friday, while the Uinta how moderate-high chances (50-70%) for the
same snowfall totals through 12Z Friday. This is likely to be the
first winter storm of the season for these mountain ranges this
season and will result in impacts residents have not dealt with in
months (poor visibility, slick/snow covered roads). The WSSI-P does
sport moderate-high chances (50-70%) for Minor Impacts in the
Absaroka, Wind River, Big Horn, and Uinta ranges, as well as some
of the tallest peaks of the Wasatch through Friday morning.


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


Mullinax




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