Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
307 PM EDT Sat Sep 28 2019

Valid 00Z Sun Sep 29 2019 - 00Z Wed Oct 02 2019

...Western U.S....

...A prolonged heavy snow event focused at higher elevations
across the eastern slopes of the northern Rockies in north-central
Montana continues through Sunday night...

The 50th percentile probabilistic snow forecasts and those from
the forecast offices indicate the likelihood of 3-4 feet of snow
in the northern Rockies of MT through Monday.

High pressure in western Canada combines with a lee trough/low
over WY/eastern MT by Saturday to provide a broad area of
northeasterly flow and cold air banked up against the eastern
slopes of the Rocky Mountain front in north-central MT. This flow
is upslope for the eastern slopes of the Rockies in north-central
MT. Moisture streaming up the Great Plains aids heavy precip rates
particularly for eastern slopes of the Rockies until shortwave
activity rounding the low/trough pushes across the northern Plains
Sunday night into Monday.

Multiple feet of snow are possible above about 4000ft in north
central MT, focused over the areas around Glacier National Park.
Low-mid level frontogenesis maxima rotate around the mid level
circulation across the ranges of WY, promoting ascent and
resulting in heavy snow in portions of the Tetons, Wind River and
Bighorn Mountains.

While confidence is high in the area of heaviest snow, the precip
type transition zone on the high Plains of northeast MT features a
tight gradient of snow accumulations, leading to more uncertainty
on accumulations. Initial conditions are too warm for snow across
southeastern MT, and with the 700 mb low tracking northwest of
this area, the primary snow areas are expected to be across
central MT.

The event winds down Sun evening as the 700 mb low tracks into
Canada, with ascent winding down in tandem with the low's
departure.

The probability of significant icing is less than 10 percent.

Petersen