Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
431 PM EDT Wed Oct 03 2018

Valid 00Z Thu Oct 04 2018 - 00Z Sun Oct 07 2018

Day 1...
...Mountain West...
Upper low off the southern California coast will open and eject
northeast into the northern Plains. Moist advection ahead of this
feature will produce mountain snows across the Rockies, but very
warm temperatures will confine the highest accumulations to
elevations above 8000 ft. WPC probabilities show the potential for
several inches of snow in the highest peaks of the Sierra Nevada,
the Uinta, and the Wind River ranges.
 
...Northern Plains... 
The filling shortwave lifting northeast into the Plains will
induce weak low pressure across Wyoming spawning warm advection
snowfall into the Dakotas Thursday evening. The probability for
snow is highest across south-central North Dakota where the
atmospheric column will initially be entirely below freezing
before slowly warming, but WPC probabilities remain low for
greater than 2 inches of snowfall.
 
The probability of significant icing is less than 10 percent.

Day 2...

...Northern Plains...
Two distinct shortwave troughs will move across the Northern
Plains on Day 2 producing the potential for accumulating snows.
The first will weaken and lift northeast very early on Friday
creating light accumulations before a more potent trough digs into
the Pacific Northwest and races into the Northern Plains during
the afternoon and evening. The NAM continues to be a cold outlier
with its thermal profiles and produces more snow locally, but the
GFS/ECMWF are in good agreement and keep the warm nose at 850mb
south of most of eastern North Dakota and northern Minnesota.
There is a robust signal for heavy snow in north/eastern North
Dakota where mid-level frontogenesis maximizes in conjunction with
strong omega through a nearly saturated dendritic growth zone, and
high probabilities exist for more than 4 inches of accumulation
with over 6 inches possible. Lower amounts are likely into
Minnesota where the warm air advects farther northward, but high
probabilities remain for 2 inches or more there.

...Northern Rockies... 
A shortwave trough lifting NE out of the area early Friday will be
immediately replaced by a more potent trough digging southward
from the Pacific Northwest. This will result in widespread
mountain snows, with the highest potential for heavy snow across
portions of the Rockies in south-central Montana and northwest
Wyoming including Yellowstone and Grand Teton National Parks. WPC
probabilities indicate a high potential for greater than 4 inches
of snow in this region, with up to 12 inches possible in the
highest terrain.

The probability of significant icing is less than 10 percent.

Day 3...

...Pacific Northwest and the Rockies...
Shortwave will dig into the Pacific Northwest and through southern
California amplifying a trough across the western CONUS while
surface high pressure lifts northeast through the Northern Plains.
Return flow around the surface high will increase moisture into
the West as the upper feature and its associated surface
reflection sink southeast along the Pacific Coast. This will
produce high elevation snows across the Rockies, Uintas, and
Bitterroots, as well as in the highest terrain of the Blue
Mountains in Oregon. WPC probabilities also indicate a good chance
for greater than 2 inches of snow in the Cascades of Washington
including Mount Rainier.

The probability of significant icing is less than 10 percent.

Weiss