Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
457 AM EDT Wed Oct 25 2023

Valid 12Z Wed Oct 25 2023 - 12Z Sat Oct 28 2023

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

An anomalous 500mb closed low with heights dropping to -2 sigma
according to the NAEFS ensemble tables will drop southeast across
the Pacific Northwest today before shearing out and becoming
embedded into the westerlies over the Northern High Plains Friday
morning. Although this will result in a weaker western CONUS
trough, an additional shortwave will pivot out of British Columbia
late in the forecast period to once again re-energize the trough
across the West. These shortwaves and associated vorticity lobes
will provide robust synoptic ascent through height falls, PVA, and
downstream divergence, combining with at times impressive upper
diffluence through a potent Pacific jet streak. This forcing will
overlap a southward sinking cold front, and the interaction of
these forcings with the low-level baroclinicity will drive waves
of low pressure eastward through the period. The combination of
this impressive lift, with increased PWs from Pacific moisture and
theta-e ridging from the Gulf of Mexico reaching above +1 sigma,
again according to NAEFS, and a slowly cooling atmospheric column
will result in waves of heavy snow from the Cascades through the
Northern Plains into the coming weekend.

D1 /12Z Wed to 12Z Thu/ will feature two areas of heavy snow. The
first will be across the Cascades where divergence downstream of
the primary closed mid-level low and some enhanced ascent east of
a surface low moving along the WA coast will tap into the higher
moisture to produce snowfall in the terrain. Snow levels during
this period are expected to fall to as low as 2000 ft, so while
additional accumulations are expected to generally be modest,
noted by WPC probabilities for 6+ inches peaking around 40%, this
could result in impactful snow to travel across the Cascade
Mountain Passes. Farther east, more significant snowfall is likely
from the Sawtooth/Salmon River ranges, east through the NW WY
mountains, and extending into the Northern Plains in ND. A wave of
low pressure developing near the Great Basin will shift eastward,
with increasing moisture advection both aloft through 295-305K
isentropic ascent and as cyclonic flow around this low taps some
of the theta-e ridge from the southeast and swings it as a weak
TROWAL northwestward. As the front sinks southward, additional
ascent will be provided through upslope flow as well, and the
heaviest snow of the event is likely near Yellowstone N.P. and
surrounding terrain on D1 where WPC probabilities for 12+ inches
reach as high as 40%, highest in the Absarokas and Wind River
ranges. Farther downstream, the guidance is still suggesting a
quasi-stationary laterally translating band of heavy snow will
advect E/NE through the High Plains of MT and into ND through
Thursday morning. This will be accompanied by impressive ascent
through deformation/fgen, and occur concurrently with a cooling
column and deepening isothermal layer beneath the lowering DGZ.
This setup combined with the WPC prototype snowband tool
indicating snowfall rates of 1"/hr suggests a narrow corridor of
heavy snow with increasing SLR to make more efficient
accumulation. On D1 in this area, WPC probabilities are 50-80% for
6 inches, with some probabilities as high as 10% for 12+ inches
depending on how this band sets up, which will result in
significant impacts to travel.

As the surface wave from D1 begins to erode, secondary low
pressure is progged to develop in the lee of the Rockies, becoming
the dominant feature by Thursday night. Cyclonic flow around this
feature will more efficiently transport moisture northward into
the Rockies and Northern Plains, renewing heavy snow across these
areas. With the first impulse moving away early Thursday, some
regions of MT/WY/ND may get a brief break in snowfall, but others
may see continuous moderate to heavy snow right into Friday
morning. This second round of heavy snow may extend as far south
as the Uintas of UT, but again the NW WY/Absarokas northeast
through the High Plains of MT and into ND are favored to
experience the most significant snowfall accumulations. For the
terrain, additional enhancement through upslope flow is likely,
and WPC probabilities D2 from the highest terrain of the Uintas
through the Wind Rivers and Big Horns are 30-40% for an additional
6+ inches. Farther east, yet another band of heavy snow, this one
progged to be slightly southeast of the first, will translate NE
near the 700mb low track, resulting in high probabilities (60-80%)
for 6 inches, and storm totals across parts of ND could exceed 1
foot in parts of eastern MT and northern ND.

By D3, most of the forcing will shift northeast into Canada
bringing an end to much of the heavy snow. However, the next
shortwave digging out of British Columbia could reinvigorate some
additional snowfall in the higher terrain from the Cascades
through the Absarokas, but WPC probabilities for an additional 4+
inches are modest, peaking at 10-20% in the Absarokas.


***Key Messages for the Pacific Northwest through the Northern
Plains Winter Storm***

-- A long duration winter storm will continue today from the
Pacific Northwest through the Northern Plains. Although heavy snow
will end in the Pacific Northwest by Thursday, periods of heavy
snow will continue across the Northern Rockies and Northern Plains
into Friday.

-- Additional snowfall of 2-6 inches at some of the Cascade Passes
today will result in difficult travel.

-- Periods of heavy snow exceeding 1â€/hr in the Northern Rockies
through Friday morning has a high chance (70%+) of accumulating to
more than 12 inches, especially in the terrain surrounding
Yellowstone National Park. These heavy snow rates combined with
gusty winds will produce hazardous travel due to snow covered
roads and low visibility, especially over mountain passes.

-- Bands of heavy snow with rates of 1+"/hr at times have a high
chance (80%+) of producing more than 8 inches of snowfall from
eastern MT through northern ND. Additionally, these bands have a
low chance (10-30%) of producing more than 18 inches in parts of
ND. Travel will become dangerous due to snow-covered or icy roads,
and poor visibility from blowing snow.

-- Temperatures will fall to 20-30 degrees below normal across the
Northern High Plains and Northern Plains Thursday through the
weekend. There is a greater than 70% chance of low temperatures
falling to 10 degrees or less Fri-Sun mornings.


Weiss