Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
413 PM EDT Mon Apr 29 2019

Valid 00Z Tue Apr 30 2019 - 00Z Fri May 03 2019

...Portion of the Mountainous West onto the north-central High
Plains...
Days 1-2...

A southern stream low is absorbed into a northern stream trough
over the Intermountain West tonight with a reinforcing northern
stream shortwave trough entering from the Pacific NW on Tuesday.
Deep SW flow ahead of the southern stream low has transported
above normal moisture (PWAT anomalies of +2.5 standard deviations)
across the southwestern CONUS. Deep layer ascent is being driven
by diffluence within the right entrance of a jet streak centered
over the Dakotas and increasing 700-600mb frontogenesis as a front
banks against the Front Range. Easterly flow behind this boundary
and ahead of a surface low enhance the frontogenesis and
orographic enhancement over the CO Rockies. Day 1 WPC
probabilities are moderate for 12 inches over higher terrain in
CO. Moderate probabilities for 8 inches also exist in the Wasatch
and Uinta of Utah and the Absaroka of WY/MT. Farther east into the
High Plains of WY/CO/NE and Black Hills of SD, diffluence within
the jet streak will spread snow northeastward, and Day 1 WPC
probabilities have increased with moderate risks for six inches
over northwestern NE and a stripe of high probabilities northeast
from the Medicine Bow Mtns along the CO/WY border to the Black
Hills.

Quickly entering the scene on Tuesday is a northern stream
shortwave diving southeast from the Pacific NW which will
reinforce moisture in the area from the Pacific. Snow levels fall
through Tuesday into Wednesday, becoming around 4000 ft in
Colorado and down to the valleys in MT/ID/northern WY. Moisture is
not as robust with this second system, but ascent remains strong.
Another round of heavy snow is likely, with Day 2 WPC
probabilities indicating a moderate risk for an additional 12
inches in the higher elevations of WY, the lower Wasatch, and
western CO. Lower snow levels and continued jet dynamics will push
snowfall well northeast from WY to northwest SD. Day 2 WPC
probabilities are moderately high for 4 inches in the High Plains
of WY and low to moderate from the northeast corner of the
Panhandle of NE, and western portions of SD into southwest ND.


The probability for icing a tenth inch or more is less than 10
percent all 3 days.


Jackson