Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
414 AM EDT Thu Oct 26 2023

Valid 12Z Thu Oct 26 2023 - 12Z Sun Oct 29 2023

...Northern Rockies through the Northern Plains...
Days 1-2...

An upper trough over the Northwest Thursday morning remains the
catalyst for a swath of moderate-to-heavy snowfall from the
Intermountain West to the Northern Plains Thursday and into
Thursday night. The right entrance region associated with a 150kt
jet streak over northern Quebec will be favorably positioned over
the North Central U.S on Thursday, providing plenty of
synoptic-scale ascent aloft. Meanwhile, a healthy integrated vapor
transport (IVT) originating as far southwest as southern
California is providing a conveyor belt of moisture over a
boundary layer that is sufficiently cold enough to support snow.
The exception is along the 850mb front oriented SW-NE across
south-central ND and northern MN where a warm nose aloft will
force precipitation to fall in the form of a wintry mix. This may
result in light icing on some surfaces and possibly slick roads
where surfaces are untreated. The upper trough will initially
continue to help produce heavy snow in the higher terrain of the
Intermountain West today. Ranges such as the Absaroka, Uinta,
Tetons, Wind River, and Big Horns likely to receive additional
6-12" of snowfall accumulations through Thursday evening. As the
upper trough emerges into the Northern Plains Thursday evening,
snowfall will pick up in intensity as stronger positive vorticity
advection (PVA) and a developing 850mb low over eastern SD focus a
narrow deformation axis over central ND, through the northern Red
River Valley of the North, and into far north-central MN.

Hourly snowfall rates above 1"/hr are possible and travel
conditions will only continue to deteriorate in these areas. The
cumulative WSSI continues to show a stripe of Moderate Impacts
from southeast MT into central ND and portions and for some
portions of the Red River of the North. In these areas, residents
can expect disruptions to daily life Thursday and into Thursday
night that include hazardous driving conditions and possible
closures. The primary impacts are the overall snowfall totals
combined with the blowing snow in the storm's wake Thursday night
into Friday. Minor to Moderate impacts are also possible in some
higher terrain of the Intermountain West, most notably the peaks
of the Uinta, Wasatch, Wind River, and Sierra Madre of southern WY.

By early Friday morning, strong high pressure to the west will
build in as the winter storm races into the Upper Great Lakes.
Periods of snow will be possible across northern Minnesota during
the early morning hours, but snow will taper off before midday,
leaving gusty winds and the coldest air of the autumn
season-to-date in its wake.

...Central Rockies & Central High Plains...
Days 2-3...

Friday evening, a pair of shortwave troughs; one over the Desert
Southwest and another digging south through the interior
Northwest, will work in tandem with a strong upper ridge over the
Southeast to form a robust 200mb jet streak over the Midwest on
Saturday (NAEFS shows wind speeds above the 99th climatological
percentile over the Midwest and Great Lakes). Strong upper level
ascent will ensue over the central Rockies and High Plains thanks
to the jet streak's diffluent right-entrance region overhead. To
the north, a dome of high pressure centered over "Big Sky" country
will anchor an expansive area of sub-freezing temperatures from
the Pacific Northwest to the Upper Midwest. Over the central High
Plains, Saturday morning will initially see S-SW flow in the
850-700mb layer that results in WAA at low levels. This coincides
with a steady diet of 850-700mb moisture flux that runs directly a
secondary cold front over the Central Rockies. By midday Saturday,
the cold front is forecast to race south and cause surface
temperatures to plunge below freezing while southwesterly flow at
700mb fosters a classic overrunning event in place.

In terms of precipitation, the strong northeasterly low level flow
running into the Front Range of the Central Rockies will coincide
with strong vertical ascent aloft. The southern WY and CO Rockies
can expect periods of snow that fall heavily at times, especially
in elevations >9,000ft. Meanwhile, the overrunning setup in the
Central Plains is ripe for icing. However, confidence in ice
accumulations are lower because it is unclear how long the icing
duration window will be. Icing could start as early as midday
Saturday and linger into Sunday morning, with a long enough
duration to support >0.1" of ice accumulation in parts of the
Central Plains. The icing footprint could stretch from the western
Corn Belt to as far south as the TX Panhandle. The probabilistic
WSSI (WSSI-P) presently shows up to 10% odds for Minor icing
impacts from the TX Panhandle to southern KS on Saturday.
Meanwhile, the WSSI-P features 20-30% probabilities of Moderate
Impacts and 50-70% probabilities for >8" of snowfall in the higher
terrain of the CO Rockies. In summary, travel disruptions are
possible from portions of the central High Plains to the Midwest
with locally heavy snowfall possible in the higher terrain of the
CO and southern WY Rockies midday Saturday into Sunday morning.

***Key Messages for the Northern Rockies through the Northern
Plains Winter Storm***

-- Additional rounds of heavy snow are forecast to persist
throughout the day on Thursday before ending on Friday.

-- Periods of heavy snow may exceed 1"/hr in the Northern Rockies
and High Plains through Friday morning. Travel may be difficult
due to snow-covered or icy roads and poor visibility in areas of
blowing snow. 

-- There is a high chance (>70%) of an additional 8 inches of
snowfall over central North Dakota through Friday morning.

-- Temperatures will be 20-30 degrees below normal across the
Northern High Plains and Northern Plains through the weekend. Low
temperatures in the teens and single digits will be common over
much of Montana into the Northern Plains. Wind chills may dip
below zero at times.

-- Dress in layers and wrap up well before going outside to
protect yourself from hypothermia.

Mullinax