Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
259 PM EST Fri Feb 14 2020

Valid 00Z Sat Feb 15 2020 - 00Z Tue Feb 18 2020


Days 1-3...

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

Organized heavier precipitation is forecast to return to the
Northwest as a low-amplitude shortwave diving southeast from the
Gulf of Alaska toward British Columbia pushes a frontal boundary
and corresponding plume of deeper moisture into the Northwest on
Saturday.  The moisture surge occurs as a result of an increasing
mid-upper level jet streaming onshore, with the 12z ECMWF showing
110-130 kt 30 mb flow in WA extending into ID by 00z Sun. Several
inches of snow should occur as coupled upper divergence/lower
convergence maxima develop near the jet and enhanced moisture
pool.

The jet persists Sat night through Sunday, with the axis drifting
south with time.  The upper divergence/lower convergence areas and
moisture plume move slowly south in tandem. The slow movement
leads to likely heavy mountain snows, with areas impacted expected
to include the Cascades, Blue Mountains, and the Rockies from
Idaho and western Montana across western Wyoming and northeast UT,
and then developing in northern CO Sun .  For the 24-hr period
ending 00Z Mon, WPC PWPF suggests local amounts of a foot or more
are likely across these areas.

By late Sunday, continued southward drift of the upper jet maxima
across CO and then east on to the Plains will drive the moisture
and lift in the CO ranges, so the expected highest
amounts/probabilities on Mon are forecast to be in CO.
The models have converged on the height forecasts 700-500 mb, so
the targeted ranges and QPF/resultant snow amounts cluster better
than average.

...Northern Plains/Upper MS Valley...
A northern stream wave crossing the northern Plains towards the
upper MS Valley leads to downstream warm/moisture advection, so
light snow should develop in the eastern northern Plains to the
upper MS Valley, then spreading towards the upper lakes late Mon
or Mon night.  A couple of inches are likely across the region.
Light icing is possible in portions of southeast SD to southern
MN, northern IA, northwest IL, and adjacent southern Wisconsin.

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

Petersen