Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
441 PM EDT Sun Oct 23 2022

Valid 00Z Mon Oct 24 2022 - 00Z Thu Oct 27 2022

...Northern/Central Rockies through the Northern High Plains...
Day 1...

An intense surface low (~985 mb) over South Dakota, coupled with
an anomalous mid-upper level low situated to its west over
Wyoming, is the main thing making winter weather headlines in the
short term forecast period.  The latest model guidance continues
to suggest multiple favorable parameters for early season moderate
to heavy snow across portions of eastern Montana/north-central
Wyoming, and extending into far western North Dakota later Sunday
night, and especially into Monday morning as the surface low lifts
towards the northeast.  The combination of strong left exit upper
jet dynamics, 700 mb frontogenesis, and the presence of a trowal
feature in the vicinity of the deformation zone will provide
strong lift and periods of enhanced precipitation rates.  Marginal
temperatures in the boundary layer and strong winds in the lower
troposphere will tend to lower SLRs and lead to a denser snow
overall, and some of these areas will likely start as rain before
the column cools sufficiently enough to support a transition to a
heavy wet snow.  Relatively warm ground at the onset may also
serve as a mitigating factor, although this can be overcome by
higher snowfall rates that could exceed one inch per hour at
times.  The majority of the snowfall is likely to occur between
6Z-18Z Monday, and then taper off as the low and its forcing lift
northward over south-central Canada by Monday evening.  Thermal
profiles will likely be too warm to support noteworthy snow east
of U.S. Route 83 in the Dakotas.

The best prospects for 6-12 inch totals through Monday evening
will be across the Big Horn Mountains and the higher terrain of
southeast Montana, and the latest forecast is slightly higher
compared to the previous forecast for this region.  Far northwest
South Dakota and areas of North Dakota near the Montana state line
have good chances of 3-6 inch totals, and some lingering snow over
the western Colorado Rockies may lead to several inches of
additional accumulation here.  A secondary low pivoting around the
base of the main trough may result in some light snow across the
higher mountains of central/northern New Mexico as well Monday
morning.



...Pacific Northwest into the Northern Rockies...
Days 2-3...

The flow pattern remains relatively active across the northwestern
quadrant of the nation going into the first half of the work week
with two separate progressive shortwaves expected to pass from the
Pacific Northwest to the Northern/Central Rockies through
Wednesday.  The first one is progged to progress inland across
Washington Monday evening and then reach the Northern Rockies on
Tuesday, followed by another potentially stronger shortwave
crossing the Pacific Northwest Tuesday night.  A cold airmass
ahead of the first shortwave is expected, and each shortwave will
likely result in even colder boundary layer temperatures and thus
lowering snow levels.  The colder airmass and strong upslope flow
with both of these events will likely lead to a drier snow with
higher SLRs compared to the storm system currently over the
Plains. 

The greatest snowfall accumulations are expected from the Bear
River Mountains of Utah to the Tetons and Wind River Range in
Wyoming, where the best potential for 8-12 inches of additional
snow exists.  This also holds true for the spine of the Cascades,
with potentially 1-2 feet of snow for the higher ridges and
summits, and 6-12 inches for the Bitterroot Mountains of Idaho.


The probability of significant icing is less than 10 percent
nationwide.

Hamrick/Jackson