Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
429 PM EDT Wed Oct 14 2020

Valid 00Z Thu Oct 15 2020 - 00Z Sun Oct 18 2020


...Northern Rockies...
Days 1-3...
Shortwave energy embedded within strong, northwest flow will
continue to support showers late Wednesday, through the overnight,
and into Thursday across the northern Rockies.  While widespread
heavy accumulations are not expected, some locally significant
totals are possible across the western Montana mountains and the
Bighorns, where snow levels are forecast to drop to 3000-4000ft
overnight.

As this energy moves downstream and upper level heights
momentarily build across the Northwest, showers are expected to
diminish by late Thursday, with dry conditions forecast to
continue into early Friday across the region.  However,
precipitation is expected to return by late Friday as another
shortwave rounding the top of the ridge dives southeast into the
region.  Developing upslope flow, along with low-to-mid level
frontogenesis will help support the potential for locally
significant accumulations along the western Montana to the
northern Wyoming ranges east of the Divide.

...Northeast...
Day 3...
A shortwave trough moving through the base of a broad-scale trough
is expected assume a negative tilt as it moves from the Ohio
valley into the Mid Atlantic states early Saturday.  This system
will continue to amplify as it lifts across the Northeast into
Atlantic Canada by late Saturday.  Surface low pressure is
forecast to develop along a stalled frontal boundary positioned
off of the Mid Atlantic coast late Friday and deepen as it tracks
north into the Gulf of Maine on Saturday.  Model differences
regarding the track and intensity of the system has limited
confidence in the details; however, a changeover to snow with at
least some minor accumulations do appear likely over parts of the
northern New England mountains on Saturday.

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

Pereira