Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
425 AM EDT Wed Oct 11 2023

Valid 12Z Wed Oct 11 2023 - 12Z Sat Oct 14 2023

...Washington & Oregon Cascades...
Day 1...

A cold front sweeping through western Washington and Oregon early
this morning along with the core of the mid-upper level low moving
onshore will continue to bring unsettled weather and widespread
precipitation to the region through the rest of today and tonight.
The impressive moisture plume and westerly flow resulting in
favored upslope will contribute to some hefty precipitation totals
but the relatively higher snow levels (6000+ ft) will mainly
confine the greatest snowfall accumulations to the highest peaks
of the Oregon and Washington Cascades where additional
accumulations of several inches to localized 1-2 ft are expected.


...North Central Rockies & Black Hills...
Days 1-2..

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 precipitation will begin later today with greater height falls
and a steady stream of moisture traversing from south-central
Montana southward toward northern Utah and the Colorado Rockies.
The moisture anomalies and strong forcing for ascent will result
in periods of moderate to heavy snow through Thursday morning, as
the colder temps aloft pour in and the upper level low settles
overhead. At this point, the model consensus is for a deepening
area of low pressure to develop in the lee of the Rockies,
wrapping significant amounts of moisture around the northwest side
of the 700 mb low, leading to a robust deformation band of
precipitation aided by favorable upslope flow. This should
generate some impressive early-season snowfall totals for the
higher elevations of the Absaroka, Wind River, Bighorn, Laramie,
and northern Colorado Rockies ranges between 00Z Thurs - 00Z Fri
where WPC probabilities for at least 8" remain 50-90 percent. In
the Black Hills, there is slightly less confidence (30-40 percent
for at least 8") but still some members in the WSE that show some
significant totals. In the lower elevations, snowfall
accumulations (potentially 1-4") will be driven by snow rate as
boundary layer temperatures are marginal.

Impacts-wise, the WSSI-P shows probabilities of 60-80% of Moderate
Impacts in the Bighorns and Laramie Mountains more specifically,
with around 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/Taylor