Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
325 AM EST Fri Jan 11 2019

Valid 12Z Fri Jan 11 2019 - 12Z Mon Jan 14 2019


...Colorado/New Mexico across the Great Plain/mid-lower MS
Valley/lower Ohio Valley and Mid-Atlantic...

A southern stream shortwave will support significant snows across
portions of the Colorado Rockies, 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. 

The trough moves east across the central and southern Rockies this
morning with significant snow continuing in the upslope region of
the Colorado Rockies west of the developing low-level cyclone.
Then as the upper trough moves east across the southern Plains
through tonight 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 advection,
this initial period of snow tonight into Saturday followed by a
second period of snow as the trailing comma-head moves through
later in the day should result in heavy accumulations across this
region. Low WPC probabilities for 12 inches are across central MO
and the St. Louis metro for Day 1. An icy mix should result in
some significant ice accumulations across southern MO and the
Ozark region of AR with Day 1 WPC probabilities for a quarter inch
moderate over south-central MO and low probabilities in northern
AR.

The system becomes flatter and more progressive aloft Saturday
night. Lighter precipitation with marginal thermal profiles is
supporting less potential for heavy snows further east along the
Ohio valley into the central Appalachians. Moderately high
probabilities for four inches are in a swath on Day 2 from
south-central IL east to southern OH and over the central
Appalachians. Will have to monitor the low level frontogenesis on
the 925mb warm front Saturday night which may produce enhanced
swaths east across the central Mid-Atlantic.

With the trend toward a flatter and more progress system moving
across the Ohio valley, the model consensus continues to have a
the coastal low developing and tracking farther south off the
Carolina Coast, resulting in lighter snow amounts for the
Mid-Atlantic and little snow for NC Saturday night and Sunday.
However, with a strong surface wedge in place significant icing
event for east from the crest of the southern Appalachians into
the adjacent Piedmont is possible. WPC probabilities show a
Moderate Risk for ice accumulations of 0.25 inch or more across
portions of western North Carolina on Day 2.

Jackson