Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
348 PM EST Sun Jan 19 2020

Valid 00Z Mon Jan 20 2020 - 00Z Thu Jan 23 2020


Day 1...

...Great Lakes...

Tonight, winds that become northerly will reorient snows south of
the Great Lakes before diminishing Monday morning as high pressure
extends across the region.  A secondary area of light snow will
occur at the south end of lake Michigan in northwest IN tonight
before ending Mon. 

Days 2-3...

...Western U.S....
An upper low and associated frontal system are expected to move
into the Pacific Northwest and northern California, bringing
widespread precipitation back into the region on Tuesday. 
Several inches of snow are expected in the higher elevations of
the WA Olympics, WA/OR Cascades,  and CA Shasta/Siskiyou, and
northern Sierra Nevada mountains.

A lead shortwave crossing the southwest moves through AZ/southern
UT and then western CO/NM.
The trough is accompanied by moisture fluxes and ascent, which
results in light snow in San Juans of CO and adjacent northern NM.

On Day 3/Wed, a second surge of moisture comes onshore in
confluent flow and produces more mountain snow in the WA/OR
Cascades, where several more inches are expected.  A few models
show potential for an additional foot in the WA Cascades. The lead
area of confluent flow and pooled moisture and lift crosses ID
into northwest WY/northern UT, with light snows along the wave's
path.


Day 3 Central US...
The models show a frontal system drifting east as a 700 mb trough
moves east from CO across the high plains.  Return southerly flow
from the southern Plains advects moisture into the frontal zone,
so precip should develop on Wed.
Out ahead of the front it should be too warm for snow.  With
heights falling aloft on Wed., precip may change from rain to
snow.  Different models have differing locations where waves may
develop along the front and produce a period of snow from the
Upper MS Valley to the central Plains.  Probabilities are low
until better agreement develops.

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

Petersen