Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
431 PM EDT Sat Oct 09 2021

Valid 00Z Sun Oct 10 2021 - 00Z Wed Oct 13 2021

...Pacific Northwest...
Day 1...

A shortwave digging along the coast of British Columbia early
Sunday will move into WA/OR while slowly filling. Pinched
confluent flow south of this feature will combine with a modest
Pacific jet streak angling onshore to drive moisture into the
region. At the surface, a wave of low pressure and its associated
cold front will advect southeastward, enhancing ascent through
low-level convergence and pre-frontal WAA. Snow levels ahead of
the front will be high within the warm sector, likely above 8000
ft, and most of the precipitation on Sunday morning will be rain.
However, as the front moves eastward, snow levels plummet to
3000-4000 ft. This will cause precipitation to changeover to snow,
with the heaviest amounts likely in the Cascades where upslope
enhancement will also occur. The system will move pretty quickly
to the east, exiting the region late D1, so WPC probabilities for
4 inches are modest except in the highest terrain including Mt.
Rainier. Some light accumulations are possible at pass level,
including Snoqualmie and Stevens, before precipitation winds down
early on Monday.


...Great Basin, Northern and Central Rockies...
Days 1-3...

An impressive early season snowfall is becoming more likely across
much of the Central and Northern Rockies.

A shortwave dropping out of the Pacific Northwest will move into
the Great Basin Tuesday morning while amplifying into a potent
closed low progged by the NAEFS ensemble to reach the
climatological minimum with respect to 700mb heights, and driving
a full latitude trough across the West. This closed low should
then advect towards the Four Corners before beginning to eject to
the northeast late D3 /Tuesday evening/. As the parent trough
sharpens, both upstream and downstream jet streaks will intensify,
and phasing of the subtropical jet stream over the Southwest will
both enhance ascent and increase moisture into the region Monday
into Tuesday. Deep layer ascent is likely to become impressive
through height falls, PVA, and robust upper diffluence in the LFQ
of this phasing jet streak leading to widespread precipitation
across the Intermountain West. At the same time, intense moist
advection downstream of this trough will surge northward across
the Plains on a 50kt LLJ, driving a strong theta-e ridge
northward, eventually lifting as a WCB into a TROWAL into WY/MT
Tuesday. At the surface, a cold front will be moving
southeastward, with periods of buckling due to waves of low
pressure developing along it. This baroclinic gradient will not
only enhance ascent, but also drive lowering snow levels on CAA in
its wake.

Snowfall will likely begin on Sunday /D1/ in the Absarokas of
MT/ID where weak deformation and modest upper diffluence will
combine to produce a few inches of snow. WPC probabilities for 4
inches are as high as 30%, generally above 7000 ft in these
ranges.

Forcing and available moisture become more impressive Monday and
Tuesday, leading to the first heavy snowfall of the season for
many. As the trough deepens and cuts off, deep layer ascent
intensifies atop increasing moist advection both from the Pacific
jet streak and from theta-e advection from the Central Plains.
Mid-level deformation near the MT/WY border will drive the axis of
heaviest snowfall D2, while temperatures and snow levels cool with
the advance of the cold front. WPC probabilities on Monday are
high for 6 inches in parts of the Salmon River, Bitterroot, and
Absaroka Ranges, with snow levels falling through the day from
7000 ft to 4000 ft.

As the trough amplifies further on Tuesday, a surface low develops
in the lee of the Rockies, leading to even more widespread and
impressive ascent. Increasing moist advection embedded within the
TROWAL will spread into WY/MT, with moist easterly flow producing
intense upslope as well. SLRs across the region are likely to be
somewhat limited in the warm/moist advection and early season
event, progged to be in the bottom 25th percentile across most of
the region. Despite that, significant QPF should still produce
heavy snow, especially on the upwind side of the Big Horns where
WPC probabilities for 12 inches are above 80%, and storm total
snowfall may reach 30 inches. In the other ranges, including the
Wind Rivers and Absarokas, WPC probabilities are high for 8
inches, and event total snowfall could reach 1-2 feet. Additional
moderate to heavy snow is likely across the Four Corners and Great
Basin ranges, although with a less focused mechanism beneath the
upper low. WPC probabilities in these areas are moderate to high
for 6 inches on Tuesday.

With snow levels falling to 3000-4000 ft, and intense ascent
likely in northern WY/southern MT, some light accumulations are
also possible into the elevated valley floors.


The probability for significant icing (0.25 inches or more) is
less than 10 percent.

Weiss