Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
404 PM EST Sat Jan 12 2019

Valid 00Z Sun Jan 13 2019 - 00Z Wed Jan 16 2019


...Central and southern Appalachians/Mid-Atlantic...

Warm advection precipitation will continue to spread east ahead a
southern shortwave currently centered over the Mississippi valley.
 Moist low level warm air moving over the top of a shallow wedge
of cold air anchored east of the Appalachians will set the stage
for freezing rain with significant icing likely along the eastern
slopes of the central and southern Appalachians into the adjacent
Piedmont region late Saturday into Sunday.  WPC probabilities
continue to show a Moderate to High Risk for ice accumulations of
0.25 inch or more across portions of western North Carolina,
southwestern Virginia and southern West Virginia.  Meanwhile, a
mixture of light snow and sleet is expected further to the into
the central and northern Mid-Atlantic states.  Snows will be come
better organized  on Sunday as the upper trough continues to drift
east and surface low pressure begins to organize and track north
along the Mid-Atlantic coast.  Models have been trending slower
with the upper wave as it moves east, allowing for the further
development of embedded shortwaves within the parent trough and a
slower surface low track with a more defined deformation zone with
heavier precipitation.  The 12Z 12km NAM was the strongest example
of this, but due to a poor initialization, appears to be
over-amplified from the onset and is likely too far north and too
heavy with its precipitation across the Mid-Atlantic on Sunday. 
The WPC preference was for a solution more similar to the 3km NAM,
GFS and ECMWF.  These models have shifted further to the north,
bringing higher probabilities for significant snow more to the
north over the Mid-Atlantic but not to the degree of the lower-res
NAM.  WPC probabilities show the greatest threat for heavy
accumulations centering from the central and northern Virginia
mountains to the DC Metro area and southern Maryland, where a High
Risk for 4-inches and Moderate Risk for 8-inches of snow are
indicated by the WPC Day 1 probabilities.  Models show the low
moving east from the Mid-Atlantic coast Sunday night with
organized precipitation ending.

...Southwest to the southern Rockies...

An upper low closing off across Southern California will move east
across the Southwest Saturday night into Sunday, undercutting an
upper ridge centered over the Great Basin.  This is expected to
support snow, with locally heavier accumulations along the
Mogollon Rim of Arizona into western New Mexico Saturday night
into early Sunday.  Snows will shift further east on Sunday, with
locally heavy totals expected along the central New Mexico
mountains.

...California...

An upper low is forecast to close off and drop southeast across
the eastern Pacific on Sunday.  This low is expected to slow as it
approaches the previously noted downstream ridge.  With the ridge
remaining in place, guidance shows the low weakening with energy
lifting back to the north across Northern California late Monday
into Tuesday.  This will support snows, with the potential for
some locally heavy totals along the Sierra.

Pereira