Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
407 PM EDT Sun Oct 20 2019

Valid 00Z Mon Oct 21 2019 - 00Z Thu Oct 24 2019

Days 1-3...

...Pacific Northwest to the Northern and Central Rockies...

A low-amplitude shortwave embedded within fast northwesterly flow
will move quickly from the Pacific Northwest late Sunday to the
central Rockies by early Monday.  This is expected to produce
mainly light snow accumulations across the region, however some
locally heavy amounts are possible, especially across the higher
elevations.

Models show the next system approaching British Columbia and the
Pacific Northwest late Monday.  While this will be accompanied by
a plume of deep moisture that will likely bring heavy
precipitation to western Washington and the northern Rockies,
increasing snow levels through early Tuesday will limit the threat
for widespread heavy snow amounts. 

A shortwave trough moving into western Canada on Tuesday will
begin to drop southeast Tuesday night, bringing colder air back
into the northern Rockies Tuesday night into Wednesday.  Behind
the cold front, some locally heavy snow may develop along the
western Montana into the northwestern Wyoming ranges Tuesday night
into early Wednesday.  Meanwhile, expect a swath of light
accumulations to follow a weak surface low dropping southeast
across eastern Montana into the western Dakotas.


...Northern Plains and Upper Midwest...

A closed upper low will continue to develop and moves east across
the northern Plains Sunday night.  East of the Black Hills,
thermal profiles suggest some periods of wet snow mixing with
mostly rain, but with little snow accumulation, as the system
moves east across the northern Plains Sunday night into Monday. 
By late Monday, models show the system beginning assume a negative
tilt and deepen.  Strong cold air advection may support a period
of snow, with an inch or two accumulation possible across
northwestern Minnesota, before precipitation ends Tuesday morning.


The probability of significant icing is less than 10 percent.

Pereira