Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
308 PM EDT Tue Oct 29 2024

Valid 00Z Wed Oct 30 2024 - 00Z Sat Nov 02 2024


...Central Rockies onto High Plains....
Day 1...

A late October winter storm is underway across the Central Rockies
and High Plains largely due to an unusually strong upper level
trough (500mb and 700mb heights as low as the 2.5 climatological
percentile in parts of the central Rockies) and strong upper level
ascent beneath dual-divergent channels of 250mb jet streaks aloft.
Vertical cross sections depict excellent 700-550mb frontogenesis
aligning over east-central Montana this afternoon and evening,
which will coincide with the the heaviest snow bands that generate
1-2"/hr snowfall rates according to the 12Z HREF. The lack of a
frigid air-mass will keep the heaviest snowfall totals to the
typical mountain ranges and where the heaviest snowfall rates ensue
across Wyoming and South Dakota's Black Hills. Farther south and
west, closer to the lower/colder heights/temperatures at 700mb,
periods of snow over the Wasatch, Uinta, and Colorado Rockies will
persist this evening but snowfall rates will gradually dissipate as
the strongest synoptic-scale forcing heads into the Northern
Plains tonight. While some lingering snow showers may stick around
in Wyoming and the Black Hills Wednesday morning, additional
snowfall will be minor as temperatures in the boundary layer warm
and moisture aiding in producing snow departs.

From 00Z this evening onward, WPC probabilities show moderate-to-
high chances (50-80%) for additional snowfall totals >8" in most
mountain ranges in central Wyoming and the Black Hills. Similar
probabilities are present but for >4" in parts of the Uinta and
Colorado Rockies. The Probabilistic WSSI between 00Z Wed -
00Z Thurs shows high chances (>70%) for Minor Impacts in the Black
Hills and along the Front Range in Wyoming. Minor Impacts will also
be felt in parts of the Central Rockies in southern Wyoming and
central Colorado. Impacts are likely to included snow covered roads
and reduced visibilities in complex terrain, making for dicey
travel in affected areas.

...Northwest to the Northern Rockies...
Days 2-3...

Following a mostly dry Tuesday in the Pacific Northwest, a potent
storm system arrives on Wednesday that will direct the next round
of unsettled weather into the region. NAEFS does show the upper
low barreling into the the Pacific Northwest and southern British
Columbia Wednesday night does sport 500mb and 700mb heights that
are below the 10th climatological percentile and a narrow IVT
channel peaking up to 400 kg/m/s that will supply modest moisture
into the region. That said, there is not much in the way of a sub-
freezing air-mass either in front of the storm or arriving in its
wake. This will keep heavy snow to elevations above 5,000ft in the
Cascades and Olympics. By Thursday morning, the upper low
continues to deepen and moves into western Oregon, resulting in
snow levels falling as low as 3,000ft in some parts of the Cascade
Range. In addition, moisture from the initial round of mountain
snow in the Pacific Northwest on Wednesday will reach the northern
Rockies where periods of light-to-moderate snow will engulf the
Sawtooth, Boise, Bitterroots, and Teton ranges. The heaviest snow
arrives on Thursday in the Oregon Cascades, northern Sierra Nevada,
Siskiyou, and Blue Mountain ranges, then late Thursday into Friday
in the northern Rockies.

WPC PWPF does show moderate-to-high chances (50-70%) for > 12" of
snowfall for the event in the Oregon Cascades while the tallest
peaks have low-to-moderate chances (30-50%) for >18". Meanwhile,
there are high chances (>70%) for snowfall >8" in the peaks of the
Blue Mountains, the Siskiyou, and northern Sierra Nevada above
6,000ft. The Probabilistic WSSI shows moderate chances (40-60%)
for Moderate Impacts in the Oregon Cascades and Siskiyou Mountains
at elevations above 5,000ft. These mountains and elevations would
have the best odds of seeing impacts that would cause impacts to
daily life that include hazardous travel and potential road
closures. Elsewhere, the other mountain ranges mentioned above are
sporting moderate-to- high chances for Minor Impacts as a result of
this impending winter storm Wednesday and through Friday morning.


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

Mullinax