Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
444 PM EST Thu Jan 10 2019

Valid 00Z Fri Jan 11 2019 - 00Z Mon Jan 14 2019


...Colorado/New Mexico to mid-lower MS Valley and lower Ohio
Valley and Mid-Atlantic...

A southern stream shortwave is expected to support significant
snows across portions of the Colorado Rockies, as well as from the
central Plains to the lower Ohio valley and parts of the
Mid-Atlantic with an icy mix along the southern edge of the snow
axis. 

Models have moved into better agreement with the 12Z run, notably
the GFS which trended further to the north with the developing
surface wave over the Plains into the Mississippi valley and the
associated warm conveyor belt spreading moisture north and west of
the low.  As the trough moves east across the central and southern
Rockies on Friday, guidance continues to show significant snow
developing in the upslope region of the Colorado Rockies west of
the developing low level-surface cyclone. Then as the upper trough
moves east in the Plains late Fri into Saturday, a developing
deformation band across the central Plains into the mid
Mississippi will produce significant snows across the region. 
Models have also been trending a little more amplified in the
northern stream, supporting a tighter deformation zone and heavier
precipitation extending back across portions of Kansas and
southern Nebraska.  Strong upper forcing is expected to help
support moderate to heavy precipitation ahead of the low, with
thermal profiles suggesting periods of heavy wet snow from the mid
Mississippi valley into the the lower Ohio valley.  Despite low
snow to liquid ratios due to the warm air, this initial period of
snow Friday night into Saturday followed by a second period of
snow as the trailing comma-head moves through later in the day may
result in some heavy accumulations across this region.  The latest
WPC probabilities show a High Risk for accumulations of 4-inches
or greater, with a Slight Risk for 8-inches from central Missouri
to central Illinois on Day 2.  Any icy mix may result in some
significant ice accumulations further to the south across southern
Missouri and the Ozark region.

With the GFS trending further to the north, it like the NAM and
ECMWF shows the system becoming flatter and more progressive
aloft.  Lighter precipitation with marginal thermal profiles is
supporting less potential for heavy snows further east along the
Ohio valley into the central Appalachians.  The general consensus
of the 12Z models showed generally light snow/ice amounts from
Ohio and eastern Kentucky into western West Virginia late Saturday
into Sunday.

With the trend toward a flatter and more progress system moving
across the Ohio valley, the model consensus shows the coastal low
developing and tracking further offshore, resulting in lighter
amounts for the Mid-Atlantic Saturday night into Sunday.  However,
with a strong surface wedge in place it still appears to be a
significant icing event for portions of the southern Appalachians
into the adjacent Piedmont.  WPC probabilities show a Moderate
Risk for ice accumulations of 0.25 inch or more across portions of
western North Carolina on Day 3.  Latest guidance shows the
greatest threat for significant snows centering across central
Virginia, where the latest WPC probabilities show a Moderate Risk
for accumulations of 4-inches or more.

Pereira