Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
325 PM EST Sat Feb 16 2019

Valid 00Z Sun Feb 17 2019 - 00Z Wed Feb 20 2019


...CA/Southwest U.S...

Shortwave energy dropping into the backside of a broad upper
trough centered over the western U.S. is expected to produce
additional heavy snow in the CA Sierra Nevada tonight into early
Sunday, as well as the mountains of Southern California on Sunday.
As the upper jet rounds the base of the trough in southern CA,
favorable difluence aloft and embedded upper divergence maxima
favors lift in the mountains of AZ and then NM and southern CO
Sunday night and Monday Each of the impacted ranges along the
route have potential for 8 to 12 inches of snow starting in the
Mogollon Rim in AZ, and spreading across the ranges of northern NM
and adjacent southern CO, such as the Sangre De Cristo Mountains.
On Monday night into Tue, the upper trough in AZ allows upper
divergence maxima to linger in NM so the ranges of NM should have
a continued threat for heavy snow. With the upper jet shifting
east on Tue, the longer duration snows should be in the Sangre
DeCristo mountains of NM and upslope areas. 2 day snow totals of
1-2 feet of snow are possible in the Mogollon Rim of AZ and again
in the Sangre de Cristo mountains of NM and possibly adjacent
southern CO.

...Northern Plains to the upper MS Valley...

Models continue to show a mid level circulation developing over
the northern High Plains
becoming negatively tilted while moving slowly southeast.  Low to
mid level convergence and mid level deformation is expected to
produce a stripe of several inches of snow from eastern Montana
southeast to southeast SD, northeast NE and western/central IA
tonight into
Sunday, with WPC Day 1 probabilities a High Risk for accumulations
of 4-inches or more from southeast Montana and southwest North
Dakota to central Iowa, where a well defined upper divergence
maxima crosses. The airmass in place is cold with projected snow
to liquid ratios in the upper teens to lower 20s to one in the
Dakotas tonight and in the upper teens to one in Iowa/northeast NE.

...Great Lakes/Ohio Valley/Northeast...

Energy over the Midwest/Ohio Valley on Sunday will become embedded
within increasingly confluent/progressive flow over the eastern
U.S., deamplifying as it moves quickly across through the
Appalachians into the Northeast late Sunday into early Monday,
before moving offshore late Monday.  Broad low level theta-e
advection and frontogenesis underneath a strong jet max aloft with
a divergence maxima aloft crossing southern NY and southern New
England will support a stripe of generally light snow amounts from
the southern Great Lakes region to central and eastern New York
and southern New England, with a wintry mix farther to the south
across portions of the Ohio valley into the central Appalachians
and northern Mid Atlantic late Sunday into early Monday. 
Further south in the Appalachians, the models have trended a bit
cooler with mixed precipitation, including sloeet and freezing
rain, expected in the developing low-mid level warm advection over
sub freezing surface areas from western NC across eastern
WV/western VA and continuing in western MD to central and
northeast PA/northern NJ, southeast NY and near coastal southern
New England. A couple of high res windows indicate potential for a
quarter inch of icing near the southern WV/VA border Sunday.

The probability of a quarter inch of icing is less than 10 percent
days 2/3.

Petersen