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

Valid 00Z Sun Sep 30 2018 - 00Z Wed Oct 03 2018


Days 1/2...

...Northern Rockies/High Plains...
Increasing low to mid level frontogenesis and upslope flow will
combine with upper divergence maxima ahead of a closed low to
support precipitation across central and western Montana, with
snow levels decreasing as colder air filters in across the region.


Guidance continues to show the heaviest snow accumulations falling
east of the Divide, centering along the Lewis Range/Glacier
National Park in northwest Montana. WPC probabilities show at
least a Slight Risk for accumulations of 8 inches or more during
the Day 1 period. 

The upper level low is forecast to weaken steadily Sunday night
into Monday. Downstream from the deamplifying wave,
light 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 and light
liquid precipitation amounts are expected to hamper the potential
for significant snow amounts. 

As the wave departs further east and a shortwave ridge begins to
build, drier conditions can be expected across much of the
northern Plains on Monday. 

The probability of significant icing is less than 10 percent.

Day 3...

The models show heights beginning to lower again as an upper level
trough builds into the Pacific Northwest Tue.  Gradual cooling
aloft opens up the possibility of snow developing in the higher
elevations of the the WA Cascades and also the northern Rockies. 
A minority of solutions indicate up to four inches of snow in
Glacier National Park, MT, but most solutions have low QPF in the
form of snow, so just a low risk of four inches is shown.

The probability of significant icing is less than 10 percent.

Petersen