Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
358 AM EST Sat Dec 14 2019

Valid 12Z Sat Dec 14 2019 - 12Z Tue Dec 17 2019

...Great Lakes and Interior Northeast to central Appalachians...
Day 1...

Surface low pressure currently lifting north over the Mid-Atlantic
is ahead of a mid-level trough that becomes negatively tilted
today as the surface low lifts out of NY State. Rapid low level
cyclogenesis increases back side precipitation that rides up the
western side of the Appalachians. Snow breaks out in the higher
elevations this afternoon and lingers in upslope areas tonight in
the near westerly flow behind the system. Lake effect snow begins
in earnest from Lake Erie overnight and off Lake Ontario Sunday.
Day 1 WPC probabilities for six inches are moderately high off the
eastern side of Lake Erie in western NY with lower guidance on the
Allegheny Plateau of WV into the Laurels of PA.


...Wave crossing the central CONUS...
Days 1-3...

...Western U.S....
A mid-level shortwave/upper jet dives southeast across California
into the Great Basin tonight and reaches the central Rockies late
Sunday. This will shift the precip focus of recent days farther
south with light to moderate snow from the Sierra Nevada and Great
Basin ranges of NV, increasing in intensity due to remnant
moisture over the ranges of UT and the central Rockies of CO (into
WY and NM). Low Day 1 probs for six inches in CA/NV give way to
low to moderate probs for 12 inches for the higher ranges of UT/CO
and the San Juans into NM. Snow tapers off across the Mountain
West behind the trough axis that crosses CO/NM Sunday night.


...Central Plains to the Ohio Valley...
Surface low pressure develops in the lee of the CO Rockies this
evening ahead of a weak leading shortwave trough ahead of the
stronger one entering CA around that time. A band of snow develops
on the northeast side of this low from warm air advection with a
swath of low Day 1 probs for four inches from east-central WY
across the Sandhills of NE.

Surface low pressure ahead of the main shortwave develops in the
lee of the northern NM Rockies Sunday morning with associated
precip developing along a convergence line across eastern CO and
KS Sunday morning. This system in fast flow quickly shifts east,
but the broad convergence zone and then comma head maintains
wintry precip over KS into MO for 24 to 30 hrs into Monday.
Thermal profiles suggest mainly snow across northern KS eastward
across northern MO, across central IL into central IN, with a
stripe of freezing rain from southeast KS, south-central MO into
southern IL Sunday into Monday.

As the upper level trough continues to move east Sunday night into
Monday, warm advection precipitation consisting a of a wintry mix
is forecast to shift east from the Mississippi through the Ohio
valley into the Appalachians. Meanwhile, additional snows will
develop on the backside of a deepening surface low as it tracks
from the mid Mississippi across the Ohio Valley and into the
Northeast on Monday. A non-NAM solution, which is farther south,
was preferred.

Jackson