Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
414 AM EDT Tue Oct 30 2018

Valid 12Z Tue Oct 30 2018 - 12Z Fri Nov 2 2018

Northern Rockies
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A long wave trough will amplify as it moves east over the western
U.S. through the middle of the week. Moist and confluent low level
flow under the jet produces bands of enhanced moisture and lift
over the ranges of Idaho and northwest Montana on Tuesday.  This
will result in light accumulating snow for the higher elevations
of the Cascades and northern Rockies.  By Tuesday night into
Wednesday, the well defined upper level jet with divergence aloft
crosses from southern British Columbia through northeast
Washington, Idaho, western Montana, and Wyoming, with heavier snow
for the Bitterroots and the mountains of northwest Wyoming on
Wednesday.

Continued shortwave activity in northwest flow over the northern
Rockies will maintain snow chances for the higher elevations of
the northern Rockies going into Thursday as Pacific moisture moves
inland and is orographically forced.  WPC Probabilities are
moderate to high for 4+ inches from the Bitterroots to the
Absaroka Mountains on Wednesday, and moderate on Thursday for
areas near Yellowstone National Park and the Big Horn Mountains.


Colorado and New Mexico
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Significant early season snow continues to be very likely across
south-central Colorado and extending into north-central New Mexico
for Tuesday night and into Wednesday morning.  An amplified trend
continues with the southern stream trough crossing the Four
Corners region, and therefore slightly higher QPF compared to the
forecast yesterday.  This amplifying trough will be combined with
upper level divergence situated over low level frontogenesis,
providing forcing for deep layer ascent. 

Cool and moist low level northeast winds developing north of the
developing surface low over southern New Mexico and south of a
surface high over Wyoming, along with PWs near half an inch over
eastern Colorado, will create a favorable upslope flow over the
Palmer Divide and the foothills, and extending across the Front
Range to include the Sangre de Cristo range. These are all
favorable parameters for early season snow at higher elevations,
and operational and ensemble models suggest a likelihood for 8 to
locally 12 inches of snow over the higher southern Colorado peaks
in the Sangre de Cristo mountains.  WPC probabilities are for 8+
inches of snow, and moderate for 12 inches.  A strong
southwesterly jet will push moisture farther northeast over the
High Plains, but thermal profiles suggest snow mainly for areas
west of the Kansas/Colorado border.

The probability for significant icing (0.25 inches or more) is
less than 10 percent for all three days.

D. Hamrick