Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
429 AM EDT Tue Oct 08 2019

Valid 12Z Tue Oct 08 2019 - 12Z Fri Oct 11 2019


Days 1-3...

...Pacific Northwest/Northern and Central Rockies/Northern
Plains...

Models show an amplifying upper trough and associated cold front
will sweep south across the northwestern U.S. Tuesday and
Wednesday - bringing sharply colder air and another strong,
early-season winter storm to portions of the northern Rockies.  
 
As the upper trough digs into the Pacific Northwest today, snow
levels are forecast to drop to around 3000 ft, with several inches
likely for higher elevations in the northern Cascades.  Later
today, snows will begin to develop and spread south along the
northern Rockies.  Strong low to mid-level frontogenesis/upslope
flow will support heavy snows developing across the northwestern
Montana ranges, with locally heavy accumulations expected within
Glacier National Park/Lewis Range.  As the upper trough/surface
front progress south across MT, rain changes to snow.

A prolonged period of strong low to mid-level frontogenesis and
upslope flow late Tuesday into early Wednesday is expected to
enhance the threat for heavy snows across the ranges of central
into southwestern Montana and northern Wyoming.  WPC probabilities
continue to show a High Risk for accumulations of 8 inches or more
from the Big Snowy and Little Belt Mountains south into the
Absaroka, Madison  and Big Horn Ranges ranges, with accumulations
of 1-2 feet possible in the mountains of south central MT.  

Meanwhile, lighter amounts are expected to spread northeast along
an axis of strong 
low to mid-level frontogenesis extending from central into eastern
Montana and western North Dakota, where WPC Day 2 probabilities
indicate a Slight to Moderate Risk for accumulations of 4 inches
or more.            
 
On Thursday, with the upper trough drifting east across the
northern Plains, the surface front drifts across the Dakotas into
MN.  A prolonged period of difluent flow aloft and upper
divergence maxima/700 mb convergence maxima lead to potential for
bands of heavy snow. There are minor timing differences that
impact the duration of precip in the form of snow.  WPC preferred
the more westerly ECMWF, ECMWF Ensemble Mean, 00z UKMET and 00z
NAM solutions - keeping the heavier snow totals in the Dakotas,
and slowing the transition to snow in MN.  There is potential for
heavy snow, with a few models indicating as much as a foot of snow
in western SD to central or eastern ND, with more possible beyond
the end of day 3 into day 4. 

The probability of significant icing is less than 10 percent Days
1-3.

Petersen