Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
431 PM EDT Mon Oct 29 2018

Valid 00Z Tue Oct 30 2018 - 00Z Fri Nov 02 2018

Northern Rockies
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A long wave trough will amplify as it moves east over the western
US tonight through Wednesday. 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 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 MT, 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 through at least the next week. Probabilities
for four inches in 24 hours increase through the day 1-3 period
with the highest probabilities on Thursday (when moderate
probabilities for eight inches are present from the WWD winter
ensemble blend).


Colorado and New Mexico
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Heavy snow is likely for the CO and NM Rockies later Tuesday into
Wednesday with another round reaching the northern CO Rockies
Thursday. A pronounced southern stream shortwave is progged to
dive southeastward from the Pacific Northwest and reach northern
New Mexico by 12Z Wednesday, preceded by significant mid-level
height falls.

An amplified trend with the trough crossing the western US through
Day 2 persists. This trough will be combined with upper level
divergence situated over low level frontogenesis and upslope
easterly flow from the central Plains, providing forcing for deep
layer ascent. A strong cold front is forecast to move through this
region early on Tuesday with surface cyclogenesis ensuing over the
central plains by Tuesday afternoon. Cool and moist low level
northeast winds developing north of the boundary, with PWs near
0.5 in 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 higher southern CO peaks in the
Sangre de Cristo mountains. WPC probabilities are moderate to high
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 for only higher
terrain.

The next shortwave pushes southeast from the northern Rockies
Thursday bringing moderate probabilities for four inches to the
northern CO Rockies.


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

Jackson