Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
335 PM EDT Mon Oct 11 2021

Valid 00Z Tue Oct 12 2021 - 00Z Fri Oct 15 2021

...First potent winter storm to bring heavy snow to portions of
the Great Basin and north-central Rockies through Wednesday...

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

A closed mid-level low embedded within a full latitude trough will
move across the Great Basin and Four Corners Tuesday, then
continue to deepen and lift into the Northern Plains Wednesday
before ejecting into Canada on Thursday. This trough is progged to
be exceptionally deep, with 700/500mb heights falling to the
climatological minimum according to NAEFS ensemble tables, and
will be accompanied by an intensifying and phasing subtropical jet
streak from the Pacific to drive robust deep layer ascent across
the region. Where this lift impinges upon the baroclinic zone
associated with a cold front dropping southeast, surface
cyclogenesis is forecast, and a rapidly deepening low is likely to
move from the Lee of the Rockies northeast into North Dakota,
while occluding as the upper features become vertically stacked.
This cyclone will be accompanied by intense moist advection as
rich theta-e air surges northward from the Gulf of Mexico in
conjunction with Pacific air rotating up from the southwest within
the upper jet streak. This will produce PWs that are likely to
reach 2 standard deviations above the climo mean, indicative of
the precipitation potential accompanying this system.

Deep layer synoptic ascent and impressive moisture will lead to
widespread precipitation from the Great Basin through the Northern
Plains, with heavy snow likely in the terrain, generally above
7000 ft. The guidance is quite well clustered with the progression
of the surface low and 700mb low through North Dakota by Thursday
morning. The current forecast is well aligned with the previous,
although SLRs were lowered just slightly to be near the 25th
percentile due to the warm/moist airmass and early season event.
The exception is across far western SD and ND, eastern MT, and
northeast WY. Here, impressive mesoscale dynamics are becoming
more concerning for a setup supporting a strong deformation band
pivoting northward the latter half of Tuesday into Wednesday. In
this region, impressive upper divergence within the LFQ of the jet
streak will help drive low-level frontogenesis collocated with the
best axis of deformation. At the same time, rich theta-e air will
be lifted cyclonically within a TROWAL north and west of the 700mb
low, providing additional ascent, with low-level easterly upslope
flow also driving omega. Where these features overlap, a band of
intense snowfall is becoming more likely. The column is thermally
marginal for heavy snow so accumulations will likely be rate
dependent, but strong ascent within the DGZ atop an isothermal
layer supports intense snow rates, which the WPC prototype
snowband probabilities indicate could reach 1-2"/hr.

As the system pulls away Thursday, snowfall will rapidly wane from
SW to NE bringing an end to the accumulating snow. Behind this
system the longwave trough lingers across the West and a secondary
shortwave will dig into the Rockies once again bringing additional
light snow to the Great Basin and Central Rockies.

Day 1 WPC probabilities for 12 inches are high in parts of the
Wasatch of Utah where Pacific moisture will be wrung out by
impressive height falls, mid-level divergence, and upslope flow
behind the surface cold front. 1 foot or more of snow is also
likely across the Absarokas and Big Horns where deep layer ascent
will maximize and moisture wrapping westward upslopes into the
terrain. D1 snowfall may reach 2 feet along the upwind slopes of
the Big Horns. More than 4" of snow is likely in the San Juans and
much of the other terrain of UT, eastern ID, and southern MT where
mesoscale dynamics will aid in bringing moderate snowfall even
into the elevated valleys.

By D2 the heaviest snow should shift into eastern WY, southeast
MT, and western SD where WPC probabilities for 6 inches are high
above 4000 ft in the Laramie Mountains and Black Hills. Storm
total in these areas could reach 12 inches in the highest terrain.
There has also been a noted increase in 2" probabilities along the
ND/MT line nearly to the Canadian border for the potential of the
aforementioned pivoting deformation band. Snowfall accumulations
may be limited, but WSSI indicates the potential for major to
extreme impacts due to snow load as SLRs will be low across this
region.

The trailing shortwave on D3 will bring additional snowfall which
may reach 4" in the higher terrain of the Uintas and Colorado
Rockies.


...Pacific Northwest...
Day 2...

A shortwave moving across the Pacific will drop onshore WA/OR
Wednesday accompanied by the LFQ of a 110kt jet streak. These
features together will produce a swath of precipitation moving
across the Pacific Northwest ahead of a fast moving cold front and
associated wave of low pressure. Snow levels are forecast to rise
towards 5000 ft ahead of the cold front, and then fall back to
3000 ft in its wake, although most of the accumulating snow should
remain above 5000 ft. Snowfall across the highest peaks of the
Cascades and Olympics could exceed 6 inches, but in general WPC
probabilities for 4 inches are less than 30%. Pass level snow may
reach 1-2 inches.


The probability for significant icing (0.25 inches or more) is
less than 10 percent Days 1-3.

Weiss