Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
317 AM EDT Sun Oct 28 2018

Valid 12Z Sun Oct 28 2018 - 12Z Wed Oct 31 2018

...Cascades and Northern Rockies...

A strong shortwave reaching the Pacific Northwest on Sunday
morning will lead to height falls and herald a pattern change for
much of the northwestern quadrant of the U.S. to begin the week. 
Steep lapse rates follow with the passage of the cold front, with
snow levels falling to around 5000 feet for the Washington
Cascades and 6000 for the Oregon Cascades.  Upslope enhancement is
expected owing to the steady onshore low level flow from the
Pacific.  As the moisture plume reaches the northern Rockies by
Sunday night and Monday morning, several inches of snow is likely
from the Absaroka Mountains to the Bitterroots with decent lift
within the dendritic growth zone and left exit jet dynamics in
place.  WPC probabilities are moderate for 4 inches of snow for
the higher mountains of the Cascades, and above 7000 feet for the
northern Rockies, especially for windward terrain

By Monday night and into Tuesday, this northern stream upper
trough departs the Rockies and crosses the northern plains.  A
second and stronger jet streak is progged to reach the Pacific
northwest by Tuesday morning.  Moist and confluent low level flow
under the jet produces bands of enhanced moisture and lift from
the northern Washington Cascades and then over the ranges of Idaho
and northwest Montana on Tuesday.  This will likely result in an
additional round of accumulating snow for the higher elevations of
the Cascades and northern Rockies during the Day 2 period Monday
and Monday night.

...Colorado and New Mexico...

Heavy snow is likely for the Front Range of the Colorado Rockies
and extending southward into northern New Mexico during the Day 3
period Tuesday night and into Wednesday morning.  A pronounced
southern stream shortwave is progged to dive southeastward from
the Pacific Northwest (currently over the Gulf of Alaska) and
reach northern New Mexico by 12Z Wednesday morning, preceded by
significant mid-level height falls.  This will likely be combined
with right entrance upper level jet dynamics from a jet streak
situated over the western High Plains.  A strong cold front is
forecast to move through this region on Tuesday with surface
cyclogenesis over the central plains by Tuesday afternoon.  Cool
and moist low level northeast winds developing north of the
boundary, with PWs near 0.5 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, and numerous GEFS and EC ensemble members are
indicating the potential for 8 to 12 inches of snow.  WPC
probabilities are moderate for 8+ inches of snow, and low to
moderate for 12 inches.

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

D. Hamrick