Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
347 PM EST Thu Feb 13 2020

Valid 00Z Fri Feb 14 2020 - 00Z Mon Feb 17 2020


Days 1-3...

...Pacific Northwest to the Northern Rockies...
A shortwave trough currently over the northeastern Pacific will
drop southeast, moving across the northwestern U.S. Thursday night
into early Friday.  This system is expected to bring several
inches of snow to the higher elevations of the Olympics and the
Washington Cascades, and east into the northern Rockies. 
The pair of 700 mb wave crossing these areas leads to an extended
duration of snow showers with potential for a foot of snow in the
WA Cascades and also locally in the ID Bitterroots and Glacier
National park in western MT.

Confluent flow Fri night and Sat pools moisture and lift, which
stream onshore and move into the ranges of the Pacific northwest
Fri night and extend inland Sat. 
The next jet maxima develops over the Pacific northwest Sat night
and streams across OR into ID/MT,
with slow changes in the axis setting up a likely long duration
heavy snow event in the OR/WA Cascades inland to
the Boise Mountains, Clearwater/Salmon River ranges, Glacier
National Park, and the Tetons of WY to the northern Wasatch in Ut.
 The good overlap of model/ensemble forecasts leads to a moderate
to high risk of a foot of snow
across these areas. 

Multiday totals of 3 to 4 foot are expected across favored
windward terrain in the ranges of the Pacific northwest and
ID/MT/northwestern WY Rockies.

...Great Lakes...
During tonight to early Fri, confluent flow crosses the Great
Lakes, with post frontal cold advection leading to moisture and
heat fluxes from Lake MI and Lake Erie, contributing to formation
of snow showers and lee shore convergence.  This produces lift in
the lee areas of southwest lower MI and southwest NY, with modest
light snow. A band of 4-6 inches is expected where slow movement
of the low level convergence supports persistence in southwest
lower MI.  A secondary area of light snow develops once the
northwest flow turns upslope in the mountains of WV.

On Sat, the return flow as high pressure moves east of the region
results in low level warm advection.  The boundary layer flow from
southwest to northeast crosses Lake Michigan.  The cross lake
trajectories moisten the airmass, with lee shore convergence
resulting in 3 to 5 inches in portions of western lower MI. As the
band of confluence moves downstream and drying aloft occurs, the
snow shower coverage/intensity winds down Sunday.

For Days 1-3, the probability of significant icing (0.25-inch or
greater) is less than 10 percent.

Petersen