Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
401 AM EST Sun Feb 23 2020

Valid 12Z Sun Feb 23 2020 - 12Z Wed Feb 26 2020


...Southern Rockies through Southern/Central High Plains...
Day 1...

A southern stream closed 700-500mb low will eject eastward across
CO on Sunday to eastern Kansas by Monday morning.
One maximum of QPF is in the CO Rockies with higher amounts in the
Sangre de Cristo mountains of southeast CO.
While the low reforms on the Plains of southeast CO, cold
advection occurs in the wake of the low. 
This cold advection may allow rain to change to snow before ending
on the Plains of CO to western KS.
Because of greater confidence of snow at higher elevations and
less confidence in lower elevations, the higher probabilities are
confined to mountainous areas of CO.

...Pacific Northwest/Northern Rockies/Northern Plains...
Days 1-2...

A closed 500mb trough digging from the Gulf of Alaska will cross
the Pacific northwest today.
A 30 mb jet maxima crosses in tandem, with well defined 300 mb
divergence crossing the WA Cascades and Olympics, leading to heavy
snow at higher elevations in windward aspects oft the mountains.
The jet max then extends inland and crosses the northern Rockies,
bring ascent and resultant moderate to heavy now to the ranges of
northern Idaho and western MT. WPC probabilities on D1 feature a
high risk for 12 inches in the Cascades, with only subtly lower
probabilities in the Olympics. WPC probabilities for day 1 plus
Day 2 feature potential for a foot in the Northern Rockies,
including the Lewis Range and Flathead National Forest, MT.

As the upper trough crosses the northern Plains, a low level front
develops.  Mid level deformation and frontogenesis supports  and
of snow from northeast MT extending into western SD and western to
central ND.
Current forecasts indicate stripes of 2 to5 inches of snow are
possible in the band, with isolated higher amounts possible.

...Upper Midwest/Great Lakes...
Day 3...

Phasing of northern and southern stream energy will eventually
result in a sprawling, slow moving, low-mid level circulation and
associated front crossing the mid MS Valley to the midwest.
Deformation maxima occur near the 700 mb circulation that
stretches from eastern Iowa across northern IL and southern WI,
with associated 850-700 mb frontogenesis.  Heavy snow is possible
where it remains cold enough for all snow in eastern Iowa across
southern WI and northern IL, possible extending east into lower
MI. 

The ECMWF and its ensemble mean nudged the QPF axis south on this
cycle, making the further north GFS 700 mb low and QPF/snow axis
in WI appear less likely.  The 00z GEFS 700 mb low and QPF axis
focused more on southern WI to the IL border area, making snow
probabilities higher there. The 06z run of the NAM brings the
heavy snow axis into the Chicago area, so this southern nudge to
the axis of QPF and snow into northern IL should be watched to see
if the trend continues.

WPC probabilities currently indicate a moderate risk for 8 inches.

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

Petersen