Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
439 PM EDT Fri Oct 05 2018

Valid 00Z Sat Oct 06 2018 - 00Z Tue Oct 09 2018

Day 1...

...Mountain West...
A surface low pressure will move onshore the Pacific Northwest in
advance of an upper trough and jet maximum. This will bring
increased moisture, as well as lift in the left exit region of
this upper jet, into the Cascades before spilling southwest
through Day 1. The upper trough will amplify and move very slowly
eastward while relative humidity remains high, so mountain snows
are likely. Total snowfall is expected to remain light, with WPC
probabilities for more than 2 inches confined to the highest
elevations of the Wasatch Range in Utah, as well as the Blue
Mountains in Oregon and the Sawtooth Mountains in Idaho.

...Minnesota..... 
Shortwave trough will be exiting Minnesota to the northeast early
on Day 1, Friday evening. Snow will persist near the international
border in the vicinity of a 700mb trough racing eastward which
will create light snow accumulations across far northern Minnesota
before dry advection commences by Saturday morning.
 
The probability of significant icing is less than 10 percent.

Day 2...

...Mountains of ID/UT/CO/WY/Southern MT...
An upper trough will amplify and cutoff across the desert
southwest while moving only slowly to the east. This spawns
surface low pressure development in the lee of the Rockies, with
deep southerly flow driving anomalously high precipitable water
into the mountains, The slow motion of these features combined
with cool temperatures beneath the upper trough will create
persistent and widespread mountain snows. Heavy snow is probable
as jet level dynamics and upslope enhancement combine to create
strong lift across the high terrain of the San Juans, Wasatch,
Unitas, and Bighorns. WPC probabilities across the highest terrain
are high for over 8 inches of snow, and over 12 inches is
possible. However, uncertainty still exists into how low snow
levels will get as the 700mb warm nose of temperatures above 0C
lift northward, especially into Colorado, and this is reflected in
lower probabilities below 8000 feet.

...Black Hills of SD into Western ND...
Upper trough across the west and surface low in the vicinity of
NM/CO will cause increasing potential for heavy snow across the
Black Hills. Anomalously high precipitable water will be focused
on E/SE low-level flow into the terrain, causing significant
upslope enhancement to already robust synoptic and isentropic
lift. The GFS is slower and further NE with its surface low
compared to the NAM/ECMWF, but has support from its ensembles.
This creates a large spread in potential snowfall amounts with the
GFS on the high end. Forecast soundings show deep saturation with
strongest omega through the dendritic growth zone and nearly
isothermal low-levels suggests good dendritic maintenance as well,
so very heavy snowfall rates are possible. For this potential and
the model spread, WPC probabilities are high for moderate snow but
low for heavy accumulations, but the potential exists for greater
than 8 inches of snow in the highest terrain. Lesser amounts are
likely northeast into ND as mid-level vorticity advects to the
northeast.

The probability of significant icing is less than 10 percent.

Day 3...

...Black Hills of SD into ND...
Low pressure near CO/NM will continue to advect anomalously high
moisture and significant snow into the terrain early on Day 3
before mid-level forcing weakens. This causes the low-level front
to shift eastward and the moist advection to shut off, bringing an
end to the snowfall. Strong omega in response to isentropic lift
and mid-level diffluence will continue the potential for heavy
snows, especially in the favored upslope regions of the Black
Hills. WPC probabilities are highest for greater than 4 inches of
snow in the high terrain, with lower probabilities northeast along
the low-level FGEN boundary to the northeast.

...Ranges of UT/CO/WY/adjacent MT...
Upper trough will remain across the Mountain West into Monday
evening. Mid level forcing due to spokes of vorticity rotating
around the mean trough, as well as jet level divergence will
support widespread mountain snows in an environment characterized
by strong low-level moist advection. The highest snow amounts are
likely in the high terrain of the Uintas, San Juans, Colorado
Rockies, and Laramie Mountains of Wyoming, where WPC probabilities
are greatest for 8 inches of snow. With the upper trough sinking
well into Arizona, some lighter snows are possible even into the
high terrain of the Kaibab Plateau and Mogollon Rim.

The probability of significant icing is less than 10 percent.

Weiss