Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
402 PM EDT Mon Oct 23 2023

Valid 00Z Tue Oct 24 2023 - 00Z Fri Oct 27 2023

...Pacific Northwest through the Northern High Plains...
Days 1-3...

--An early season significant winter storm continues to appear
more likely beginning tonight with multiple rounds of moderate to
heavy snow possible--

Winter is indeed coming to the Cascade Range on east to the
Northern Rockies and High Plains. A potent closed upper low diving
south towards the Pacific Northwest Tuesday morning will be
favorably timed with the arrival of a cold Canadian air-mass
surging south through the Canadian Prairies. The 500mb low
according to both NAEFS and the ECMWF ENS dips below the 1st
climatological percentile. Meanwhile, in wake of the cold front
racing south, 850-700mb temps will be below the 10th
climatological percentile. These suggest an unusually strong
synoptic-scale driver aloft in the upper low as well as a
sufficiently cold air-mass to support winter weather. The
aforementioned closed upper low is responsible for exceptional
diffluent flow aloft, fostering strong upper level ascent. In
addition, the upper low will escort a strong integrated vapor
transport into the Northwest and the Northern Rockies. NAEFS shows
IVT values as high as the 97.5 climatological percentile, implying
there will not only be sufficiently cold temperatures Tuesday
night into Wednesday, but also a steady stream of Pacific
moisture.

These key factors: sub-freezing temperatures, excellent synoptic
scale lift, and a healthy stream of moisture will remain in place
through mid-week as the upper trough in the Northwest takes its
time exiting the region. This results in a multiday heavy
snowstorm for parts of the northern Rockies and High Plains. The
snowfall begins in the Cascades and Olympics where snow levels
will plummet to as low as 1,500ft. This should lead to
accumulating snow within some of the passes of the Cascades. As
700mb moisture overruns the now sub-freezing boundary layer temps
in the northern Rockies and High Plains Tuesday evening, periods
of snow will envelope the region. Finally, strong frontogenesis
over western ND will give rise to a band of heavy snowfall from
eastern MT into western ND.

The heaviest snowfall is likely to setup in the Cascades,
Absaroka, and Tetons where 24-hr snowfall probabilities for >12"
are as high as 50-70%. From Great Falls, MT on east through the
High Plains, snowfall probabilities >6" surpass 70% to as far east
as the MT/ND border. The probabilistic WSSI shows 60-80%
probabilities for Moderate Impacts in the Cascades (80-90% for
Minor Impacts in the passes along I-90) with an expansive area of
60-80% Moderate Impacts from I-15 southwest of Great Falls on east
through the heart of Montana. There are some 20-30% probabilities
for Major impacts, but presently, snowfall totals are not high
enough (and winds are not strong enough) to raise probabilities
above 50% thus far. It is worth noting much of I-94 that stretches
from Billings on east to the ND/MT border sport 40-60% odds for
Moderate Impacts Wednesday and into Thursday. Impacts would
include closed or impassable roads and significantly reduced
visibilites. Perhaps most notably for the northern High Plains,
this is their first significant winter storm of the season and
residents impacted should have the necessary preparedness items.
For more info, visit Ready.gov and visit "Winter Weather" for more
information.

The probabilities for ice accumulations >0.10" are less than 10%.

Mullinax