Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
350 PM EDT Thu Oct 15 2020

Valid 00Z Fri Oct 16 2020 - 00Z Mon Oct 19 2020


...Northern Rockies...
Shortwave energy currently shearing across the top of a strong
ridge extending from the eastern Pacific into the Gulf of Alaska
is forecast to dive southeast across western Canada on Friday, and
is expected to reach the northern U.S. Rockies by early Saturday. 
Associated low pressure dropping southeast out of Alberta, along
with a trailing baroclinic zone, will support of the return of
organized precipitation across the northern Rockies and High
Plains by Friday afternoon.  Even as the initial wave moves
downstream across the Plains, lingering low-to-mid level
baroclinicity and upstream upper level energy will continue to
support periods of precipitation through the weekend.  Heaviest
precipitation through late Saturday is expected to center along
and east of the Divide across western Montana, with most areas
changing over to snow as cold air spreads south.  Heaviest snow
accumulations through late Saturday are expected along the
northwestern Montana ranges, including the Lewis and Flathead,
where WPC PWPF indicates accumulations of 6-inches or more are
likely.  Locally heavy accumulations are also expected farther
south across the Crazy Mountains and into the northern Absaroka. 
As precipitation continues across these areas into Sunday, models
show an upstream wave dropping farther west.  This is expected to
support better organized precipitation west of the Divide, as
evidenced by increasing probabilities for significant snowfall
accumulations along the Whitefish and Swan ranges by end of the
period.  

...Northern Plains...
Low pressure dropping southeast from Alberta into the U.S. High
Plains Friday evening, is forecast to move east across South
Dakota overnight into early Saturday.  Low-to-mid level
frontogenesis, in addition to favorable upper level forcing, is
expected to support a period of organized snow north of the low
across North Dakota into northern Minnesota late Friday into
Saturday.  Still a fair amount of uncertainty, however latest
guidance suggests heaviest accumulations are likely to center
across northeastern North Dakota, where mesoscale banding may
produce several inches.  Latest WPC PWPF indicates snowfall
accumulations of 4-inches or more are likely across this area.

...Northeast...
Models continue to show low pressure quickly developing and moving
north along the Northeast coast as an amplifying shortwave moves
from the Ohio valley into the northeastern U.S. Saturday morning. 
This system is expected to bring widespread, much-needed
precipitation to the region.  Precipitation will be mainly in the
form of rain.  However, cold air on the backside of the system
will support a changeover to snow before the precipitation ends
across portions of the Adirondacks and northern New England
mountains, with light accumulations likely across northern New
Hampshire into western Maine.

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

Pereira