Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
330 PM EST Tue Jan 11 2022

Valid 00Z Wed Jan 12 2022 - 00Z Sat Jan 15 2022

...Pacific Northwest into Northern Rockies...
Days 1-3...

A quasi-stationary low level frontal boundary stretched across
Washington state through Wednesday morning, along with steady
onshore flow ahead of the next storm storm, will provide a focus
for mainly high elevation snow for the Cascades, and extending
eastward to the Chimney of Idaho and far northwestern Montana
through Wednesday evening.  By Thursday, a second round of snow
showers are likely across the higher terrain of the northern
Rockies as shortwave energy from the main low pressure system
entering the Pacific Northwest generates lift across this region. 
This shortwave impulse then dives southeastward across the
Colorado Rockies and results in patchy areas of moderate to
locally heavy snow showers for the Day 3 period Friday.  Total
snowfall in the vast majority of cases is expected to be less than
six inches, with the exception of the highest peaks of the
Cascades where a foot or more of snow is within the realm of
possibility.

At the same time, shallow cold air over the Columbia Basin and
periods of light precipitation spilling over the Cascade crest
will allow for light freezing rain/drizzle accumulations to occur
into Wednesday with generally a tenth of an inch or less of ice
accretion.

...Northern Plains/Upper Midwest...
Day 3...

A shortwave trough entering the Pacific Northwest/western Canada
on Thursday is expected to move over the top of an upstream ridge,
sliding southeast across the northern Plains on Friday.  Models
are in generally good agreement, indicating that well-defined
low-to-mid level frontogenesis will help support a band of
moderate to locally heavy snow developing and shifting south from
the eastern Dakotas and western Minnesota into Iowa on Friday. 
WPC probabilities for snow accumulations of 4 inches or more are
now in the 30-70 percent range from eastern North Dakota to
northern Iowa on Day 3.  Southwest of the axis of heavier snow,
there is some signal for mixed precipitation, including freezing
rain.  However, widespread heavy ice accumulations are not
expected at this time. 


Hamrick/Pereira