Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
406 AM EDT Sat Sep 29 2018

Valid 12Z Sat Sep 29 2018 - 12Z Tue Oct 02 2018


Days 1/2...

...Northern Rockies/High Plains...
Shortwave energy dropping into the base of broad upper trough will
help drive colder air across the northern Rockies and High Plains
on Saturday.  Increasing low to mid level frontogenesis and
upslope flow will support an increasing chance for precipitation
across central and western Montana, with snow levels decreasing as
colder air filters in across the region.  Overnight guidance
continues to show the heaviest snow accumulations falling east of
the Divide, centering along the Lewis Range in northwest Montana,
where WPC probabilities show at least a Slight Risk for
accumulations of 8-inches or more during the Day 1 period (ending
12 UTC Sunday).  As a weakening Pacific mid-upper level low moves
into the Northwest, precipitation is expected to spread further
east across central into eastern Montana and western North Dakota
late Saturday into Sunday.  Some light snow accumulations are
possible across portions of central into eastern Montana and
western North Dakota, however marginal boundary layer temperatures
at the onset and subsequent warming is expected to hamper the
potential for significant amounts.  As the Pacific system moves
east and a shortwave ridge begins to build, drier conditions can
be expected across much of the region on Monday.

The probability of significant icing is less than 10 percent.

Day 3...

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

Pereira