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FOUS11 KWBC 161845
QPFHSD

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
245 PM EDT Thu Oct 16 2025

Valid 00Z Fri Oct 17 2025 - 00Z Mon Oct 20 2025

...Northern/Central Rockies...
Days 1-3...

The mid-level pattern becomes increasingly progressive through the
weekend as the closed low which has brought prolonged unsettled
weather to the region fills. As this feature becomes embedded in
the more general westerlies by Friday night, it will leave pinched
W/NW flow in its wake, through which two additional disturbances
will race, producing transient periods of moderate to heavy
snowfall across the higher elevations.

On D1 /Thursday night and Friday/ some lingering mid-level moisture
will remain across the terrain of NW WY in response to a departing
surface low and increasing NW mid-level flow. While moisture is
expected to erode quickly during this period, sufficient upslope
flow into this moisture will result in periods of light to moderate
snowfall before the DGZ dries and snow ends, especially across the
Tetons and near Yellowstone NP, where WPC probabilities for an
additional 2+ inches of snow are as high as 30-50%.

After this first wave ejects to the east, a secondary shortwave
will dive southeast rapidly on its heels. This impulse is likely to
dig from southern British Columbia around 00Z/Friday to the
Central High Plains by 00Z/Saturday. A pinched vorticity streamer
accompanied by rapid sharpening of the 700mb wave will drive
intense ascent, albeit of short duration, from the Northern Rockies
through the terrain of WY. Although forcing will be quick,
favorable overlap of omega into the DGZ combined with modest 0-2km
fgen will likely result in a fluffier-than-normal snowfall with
SLRs likely above climo but with briefly intense rates above 1"/hr.
This will result in a few inches of snowfall as reflected by
WPC probabilities that reach as high as 50% for 4+ inches across
the Absarokas and Big Horns. Gusty winds will likely accompany
this wave and some low-end probabilities (10-20%) for moderate
blowing snow impacts from the WSSI-P move across the area during
this time as well, indicating a threat for convective snow bursts
or even an isolated snow squall Friday night.

Thereafter, yet another wave, the third in the series, will
approach the Pacific Northwest Coast and then dive into the
Northern Rockies by the end of the period. This system will be more
impressive with impressive height falls digging out of British
Columbia leading to robust downstream IVT surging into the region
(>80% chance for 250 kg/m/s reaching well into the interior West).
With the accompanying WAA, snow levels will rise steadily to
7000-8000 ft, but then crash with the accompanying cold front to
around 5000 ft by 00Z Monday. This will result in varying snow
levels with generally below climo SLR producing impacts due to snow
load as reflected by the WSSI-P, but additional snowfall before the
end of D3 is expected to be light and confined to the Northern
Rockies near Glacier NP.


...Pacific Northwest...
Day 3...

A strong upper low (500mb heights dropping below the 10th
climatological percentile according to NAEFS) will dig along the
British Columbia coast driving height falls into the Pacific
Northwest on Sunday. These height falls will be accompanied by
vorticity streamers/PVA, and increasing LFQ diffluence as a jet
streak pivots southward upstream of the primary trough axis.
Together, this will result in increasing ascent into a moistening
column thanks to IVT reaching above 500 kg/m/s (>80% chance) into
coastal WA and OR. The result of this overlap will be an expanding
area of precipitation, with snow likely becoming more widespread as
snow levels fall through the day. Initially, snow should be
confined to just the highest terrain of the Northern Cascades, but
by the end of D3 snow levels fall to around 5000 ft (NBM 10th/25th
percentiles seem reasonable due to strong ascent and steepening
lapse rates) leading to more widespread snow and at least some
impacts at the passes. WPC probabilities on Sunday at high (>70%)
for more than 4 inches in the Northern Cascades above 6000 ft and
in the higher peaks including Mt. Rainier.


The probability of significant freezing rain across CONUS is less
than 10 percent.


Weiss


$$