Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
418 PM EDT Thu Sep 27 2018

Valid 00Z Fri Sep 28 2018 - 00Z Mon Oct 01 2018


DAY 1...

...Northern Rockies/High Plains...
Models show shortwave energy embedded within northwest flow with a
300 mb jet axis extending northwest to southeast across MT and
east across SD tonight.  Upper level divergence maxima in the
right jet entrance will combine with low to mid level
frontogenesis along with upslope flow to produce precipitation
across the region, with decreasing snow levels as colder air moves
into the region tonight.  The heaviest amounts are expected to
fall along the WY Big Horns, with WPC Day 1 probabilities (ending
12 UTC Fri) indicating a Slight Risk for accumulations of 8-inches
or more.  Several inches are expected further east across the
higher elevations of the Black Hills of SD.

The probability of significant icing is less than 10 percent.

DAY 2...

A relative lull in precipitation occurs as the upper jet weakens,
along with the strength of low level convergence within the
frontal zone.  Light snows may occurs in the mountains of
northwest MT, with a few inches of snow accumulation expected.

The probability of significant icing and/or snow is less than 10
percent.

DAY 3...

...Northern Rockies...
Precipitation returning to portions of the northern Rockies around
midday ahead of a shortwave trough digging through western Canada.
 Increasing upslope flow with low to mid level frontogenesis, with
the 12z nam/gfs showing 700 mb warm/moist advection over the top
of the low level cold air, will support another round of
precipitation with decreasing snow levels Saturday into early
Sunday.  Heaviest snow accumulations are expected to center along
the front range in northwest Montana and Glacier National Park,
with WPC Day 3 probabilities for Sat night-Sunday indicating a
Moderate Risk for accumulations of 8-inches or more.

The probability of significant icing is less than 10 percent.

Petersen