Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
324 PM EDT Tue Oct 10 2023

Valid 00Z Wed Oct 11 2023 - 00Z Sat Oct 14 2023

...Washington & Oregon Cascades...
Days 1-2...

An exceptionally amplified upper low barreling into the Pacific
Northwest will direct a conveyor belt of Pacific moisture at the
Cascade Range tonight and into early Wednesday. NAEFS shows by
between 00-06Z Wed, 500-850mb heights over the Pacific Northwest
coast are below the 2.5th climatological percentile while the
westerly IVT of the Pacific Northwest coast peaks above 500
kg/m/s, ultimately topping the 90th climatological percentile. The
brisk westerly flow oriented orthogonally to the mountain range is
ideal for enhanced precipitation rates via strong upslope ascent.
However, the lack of sub-freezing temps aloft will keep heavy
snowfall amounts confined to elevations >6,000ft. Snow will
conclude over the WA Cascades first by Wednesday evening, then as
the upper low advances farther east into the northern Rockies
Wednesday night, the westerly IVT will shut off and end the OR
Cascades period of high elevation snow. WPC 24-hr probabilities
for >6" of snowfall are highest (60-80%) over Mt. Rainier and the
Three Sisters peaks of western OR.

...North Central Rockies & Black Hills...
Days 2-3...

The upper trough responsible for the heavy snowfall in the
Cascades will also be responsible for the first significant winter
storm of the season in the North Central Rockies. The first round
of snow begins in the Absaroka and Teton Ranges as 500-700mb
height falls and a steady stream of 700mb moisture flux ascends
and traverses these ranges. NAEFS shows 700mb heights are below
the 2.5 climatological percentile Wednesday morning, displaying
this is an unusually deep mid-upper level trough overhead. Farther
south, both NAEFS and ECMWF situational awareness tools are
showing IVT values above the 90th climatological percentile aimed
at the Colorado Rockies. With a strong vapor transport present,
strong vertical ascent via orographic lift should result in round
of heavy snow Wednesday night into Thursday, especially as height
falls and colder temperatures aloft work their way over the region
Wednesday night and into Thursday morning.

The biggest factor in determining where the heaviest snowfall
occurs comes down to the evolution and track of the emerging
850-700mb mean low in the North Central Plains. The GFS is faster
in its development with a stronger deformation axis showing up
from eastern WY to the Black Hills and western Nebraska on
Thursday. The ECMWF is slower both in its deepening of the 700mb
low and in its progression east, which aligns itself in the
Canadian guidance suite. The one thing guidance agrees on is the
mountain ranges of central and eastern WY to the Black Hills of SD
have the best odds of seeing the warm conveyor belt around the
northwest flank of the 700mb low to generate a robust deformation
zone aloft and support strong orographic ascent into these ranges
on Thursday. WPC probabilities are 50-80% for 24-hour snowfall
totals >8" for elevations >7,000ft in the Absaroka, Wind River,
Bighorn, Laramie, and northern Colorado Rockies ranges between 00Z
Thurs - 00Z Fri. In the Black Hills, WPC 24-hour probabilities for
>8" of snowfall are lower (closer to 30-40%), but there are some
members of the WSE that show 10-20% probabilities for >12" in
highest peaks of the Black Hills. In the valleys of the
Intermountain West, it is possible areas as low as 6,000ft could
see totals of 1-4", but this is highly dependent upon the
thermodynamic profiles at lower levels and both the
strength/positioning of the deformation axis. Given the lack of
sub-freezing temperatures within the boundary layer at elevations
<6,000ft from the valleys of the Intermountain West to the North
Central Plains, snowfall rates will have to be prolific to
overcome marginal BL temps.

Impacts-wise, the WSSI-P shows probabilities of 40-60% of Moderate
Impacts in the Bighorns and Laramie Mountains more specifically,
with 20-30% probabilities also depicted in the Black Hills and
Wind River Range. This in turn could mean treacherous travel and
hazardous conditions for outdoor activities in these areas
Thursday and into early Friday. In western Nebraska and southwest
South Dakota, strong wind gusts combined with periods of snow
would result in reduced visibilities. This is accounted for in the
WSSI which depicts Moderate impacts are possible from the Laramie
Range on east to western Nebraska on Thursday.


Days 1-3, the probability of significant ice accumulation across
the contiguous U.S. is less than 10 percent.

Mullinax