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

Valid 12Z Fri Oct 05 2018 - 12Z Mon Oct 08 2018

Day 1...

...Cascades in Washington to Mountains of southern ID...
The approach of a surface low pressure in advance of the
approaching upper trough will advect Pacific moisture into
Washington state, continuing into southern ID the latter half of
Day 1.  This will produce elevation snows in the Cascades with a
few inches of snow possible in the highest terrain, continuing
into the Boise Mountains of ID.

...North Dakota/northern MN..... 
Snow occurring across northern ND the last several hours moves
into northeast ND and northwest MN with an initial round of warm
advection near the 700 mb front and then low level convergence in
advance of the approaching 700 mb trough. The steady forward
progression of the trough will limit duration of snowfall with
activity later today departing north of the Canadian border.
 
The probability of significant icing is less than 10 percent.

Day 2...

...Mountains of ID/UT/southern MT/WY/UT/CO...
The upper trough is forecast to amplify across Nevada and UT with
the slow moving 700 mb front allowing large areas of enhanced
moisture in the frontal circulation with vertical velocity maxima
near the front supporting several inches of snow in the ranges
from southern ID across western WY, UT, and possibly into western
CO.  The highest probability for significant snow  exists across
the higher terrain of the Uinta and Wind River ranges.  The models
indicate a closed 700 mb low forming over UT with lowering
heights/snow levels late Sat night. 

The probability of significant icing is less than 10 percent.

Day 3...

...Ranges of UT/CO/WY/adjacent MT...
The upper trough is forecast by most models to contain a slow
moving closed 700 mb low with a few models redeveloping the low
along the trailing front, as well as a few which develop
circulations along the front. At the surface, low pressure will
develop in response to this trough near the four corners region,
driving moist advection on increasing low-level southerly flow.
Mid and upper level forcing due to the low level frontal
circulation and upper divergence will produce strong lift,
enhanced by the moisture fluxes into the higher terrain of
Colorado and Wyoming. Widespread mountain snows are likely with
several inches expected in the highest terrain of the San Juan,
Uinta, and Bighorn mountains.

...Northern Plains...
A deep layer front drifts east across the northern plains in
conjunction with an upper level jet streak. 
Lift in the vicinity of the front and jet results in mixed precip
and possibly a period of snow from southeast MT and northeast WY
across portions of western SD/ND. The highest probability for
greater than 4 inches of snow focused across the terrain of the
Black Hills. The model QPF and temperature profile have enough
differences model to model and run to run to result in a spread of
several inches in these areas.

The probability of significant icing is less than 10 percent.

Petersen