Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
444 PM EDT Sun Oct 18 2020

Valid 00Z Mon Oct 19 2020 - 00Z Thu Oct 22 2020


...Northern Rockies/High Plains...
Shortwave energy embedded within progressive, northwesterly flow
aloft will continue to support periods of unsettled weather across
the region through the early part of the week.  Energy dropping
southeast across western Canada into the northwestern U.S. is
expected to support precipitation into Monday across the region. 
Early in the period, cold air in place to the north of a stalled
frontal boundary is forecast to support snow across the higher
elevations of the northern Rockies into the High Plains. 
Meanwhile, warmer air advecting from the west will likely limit
snow amounts across the lower elevations west of the Rockies. 
Then snow levels will likely increase from southwest to northeast,
limiting the potential for additional significant accumulations
east of the Rockies on Monday.  For the Day 1 period, ending 00Z
Tuesday, heaviest snow accumulations are expected to fall across
the higher elevations of the western and central Montana to the
northern Wyoming ranges -- where WPC PWPF continues to show high
probabilities for local accumulations of 6-inches or more.  In
addition to snow, models continue to signal the potential for
freezing rain, with minor ice accumulations possible across
portions of central and southeastern Montana. 

Warmer, drier air should preclude the threat for widespread
significant snow and ice accumulations across the region on
Tuesday before the next system begins to impact the region by
early Wednesday.  Probabilities for significant snow accumulations
begin to increase once again along the northwestern Montana ranges
as the next well-defined shortwave begins to dive southeast into
the Northwest.

...Northern Plains and Upper Midwest...
Shortwave energy moving east of the Rockies is expected to amplify
over the northern Plains on Tuesday.  A weak surface reflection
along with favorable upper forcing will help support a wide swath
of light to moderate precipitation across the northern Plains and
Upper Midwest.  Cold air in place is forecast to support
accumulating snow, with mostly light amounts (less than 4-inches)
expected, from northern South Dakota and North Dakota eastward to
northern Minnesota and Upper Michigan.  However, a low-to-mid
level frontal band lifting across the region may help to support
some heavier totals across central to northern Minnesota and
northern Wisconsin.  Meanwhile, southerly flow may also support
some lake-enhanced totals in the higher terrain along Minnesota's
North Shore.

For Days 1-3, the probability of significant icing is less than 10
percent.

Pereira