Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
440 PM EDT Sat Oct 06 2018

Valid 00Z Sun Oct 07 2018 - 00Z Wed Oct 10 2018

Day 1...

...Ranges of ID/MT/WY/UT/CO...
An upper level trough will amplify over the Great Basin in
response to a shortwave digging into its base. This will initiate
low-level cyclogenesis with warm advection occurring east of the
low into CO and WY. Synoptic forcing due to vorticity advection
and upper diffluence will combine with increasing isentropic lift
to produce widespread mountain snows. Warm temperatures of above
0C at 700mb flooding into much of CO and southeast WY will limit
snow to the highest terrain. WPC probabilities for greater than 4
inches of snow are highest NW of the 700mb low into the Uintas of
UT and Bighorns in WY, with lighter amounts likely across much of
the other mountain ranges.

...Western SD into southwest ND...
Sharpening 500mb trough will induce a closed 850-700mb low to
funnel warm moist air into the High Plains the latter half of Day
1. A shortwave will eject from the mean trough to lift northeast
and overhead the western Dakotas coincident with strengthening
mid-level frontogenesis and 700mb deformation to provide a brief
period of robust omega. The forcing will be of relative short
duration as the shortwave lifts quickly northeast, so WPC
probabilities are low for more than 2 inches of snow, with the
best chance in the high terrain of the Black Hills.
 
The probability of significant icing is less than 10 percent.

Day 2...
...Mountains UT/CO/WY/AZ...
Anomalously strong upper trough will cutoff over the desert SW and
migrate slowly eastward Sunday. 700mb reflection of this low will
move concurrently with the upper trough, driving warm moist
advection into the Mountains. The 700mb temperatures climb well
above 0C east of this low, so some uncertainty remains in the
eastern extent of snowfall in Colorado, but to the north and west
the potential exists for significant accumulation of greater than
6 inches in the high terrain of the San Juans, Uintas, and
Laramies. Lesser amounts are likely below 6000 feet.

...Western SD into Central ND...
Warm moist advection ahead of a closed mid-level low will combine
with upper diffluence to produce lift and snow into the High
Plains. A spoke of vorticity ejecting northeast from the amplified
upper trough will combine with 700mb deformation to spawn a region
of snow lifting across the Dakotas with light snow accumulations
possible. In the terrain of the Black Hills, subtle upslope
enhancement will increase snowfall potential, and WPC
probabilities are highest in this terrain for up to 4 inches of
snow.

The probability of significant icing is less than 10 percent.

Day 3...

...Ranges of UT/CO/WY/northern NM...
Upper trough will gradually fill and begin to eject eastward in
response to a potent shortwave and jet maximum digging into the
Pacific Northwest. Weak vorticity advection and increasing upper
diffluence in the left exit region of the approaching jet will
support light to moderate mountain snows. The highest WPC
probabilities are across the high Rockies in Colorado as well as
the Uintas in Utah, but much of the high terrain has the potential
to see accumulating light snow.

...Pacific Northwest mountains into western MT...
300mb jet maximum will push southeast from Canada, inducing lift
within the left exit region as it drops into the Pacific
Northwest. Precipitation will develop in response to this synoptic
forcing, initially across the Cascades of Washington before
shifting into the northern Rockies. The greater snowfall is
expected across the Rockies in the vicinity of Glacier National
Park, where 6" of snow is possible above 7000 ft.

...Western High Plains...
Upper trough will gradually deamplify and lift northeast spawning
cyclogenesis across Texas. Deep layer warm advection ahead of this
low will increase moisture into the plains and as the low
strengthens there is potential for increasing snow within the
deformation zone of the developing baroclinic leaf across portions
of Nebraska and South Dakota. Strong lift is possible, but models
still diverge considerably into timing and location of best QPF so
WPC probabilities remain low for more than 2 inches of
accumulation. 

The probability of significant icing is less than 10 percent.

Weiss