Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
514 PM EST Sun Feb 14 2021

Valid 00Z Mon Feb 15 2021 - 00Z Thu Feb 18 2021


...Southern Plains to the Mississippi Valley, Great Lakes, and
Northeast Days 1-2...

A major winter storm is expected across the southern Plains to the
MS Valley, Great Lakes, and Northeast.
The models show a 700 mb trough crossing east from the Southern
Rockies onto the plains will amplify overnight into Monday, with a
closed low developing. Low-mid level convergence east of the
wave/low is expect to produce an area of snow from central TX/OK
east across southern MO/AR, western KY/TN, then southern IL/IN
Monday afternoon. WPC PWPF shows high probabilities for snow
accumulations of 4 inches or more extending from Northeast Texas
across eastern AR into southeast MO, western KY, and southern
IL/IN.

Downstream from the snow area will be a precip type transition
zone consisting of sleet and freezing rain across southeast TX to
LA, eastern AR, western TN, and central KY. The models forecast a
band of significant freezing rain from southeast LA across MS to
central TN and eastern KY. A quarter inch of icing will be common
in these areas. The probabilities for a quarter inch of icing are
highest in southeast LA across central to northeast MS, northwest
LA, central TN, and southeast KY.

The stronger/more amplified 700 mb wave/closed low heads towards
the Great Lakes on Day 2.
The stronger southerly flow east of the low advects more moisture
north across the Ohio Valley and Great Lakes.
This has lead to a significant shift of the snowfall north into
southern Lower MI and off the southern shore of Lake MI from
Chicago to northern IN. 

Moderate to heavy snows will continue to move northeast through
the northern Ohio Valley into the Lower Great Lakes, and
western/northern New York and northern New England into Tuesday.
The highest probability for 4 inches of snow is across
western/northern NY, northern VT.NH/interior ME.

A wintry mix, including accumulating ice extends eastern KY across
WV, western MD, much of southern PA, northern NJ, southeast NY,
and southern New England. The highest probability for a quarter
inch of icing on Day 2 is across western MD, south central PA,
northern NJ, the lower Hudson Valley of NY, and southern New
England.

The event tapers as the system moves into the Canadian Maritimes
late Tuesday.

...Pacific Northwest to the Southern Plains...
On Day 1, a quickly moving 700 mb shortwave in northeast Pacific
moves onshore, with an influx of moisture advection and ascent
that progresses across the Pacific northwest and northern Rockies.
A secondary surge arrive slate Monday, so the 2 rounds will likely
lead to heavy snow in the WA Cascades, and secondary lower maxima
are expected across the OR Cascades inland to the Blue Mountains
and ranges of ID, northern
NV and UT to western WY.

On Day 2, the continuation of moist confluent flow at 700 mb with
enhanced moisture and ascent streams from northwest to southeast
across WA/OR/ID down into western WY/UT/CO, reaching NM by the end
of Day 2. Another 1-2 feet of snow are possible in the WA/OR
Cascades.
The snow should wind down on Day 3 in the northwest as a 700 mb
ridge approaches form the Pacific and drier air overspreads the
region.

On Day 3 Tue night-Wed., the 700 mb trough crosses from the
Rockies on to the High Plains. Snow extends form the ranges of NM
and then develops across the TX panhandle and across OK to the
ArklaTex. A mixture of sleet and freezing rain is expect south of
the snow area in eastern TX to northern LA and eastern AR.  The
WPC PWPF indicates increasing probabilities for the Sangre
deCristo Mountains of northern NM/adjacent southern CO and then
from the Texas Panhandle across Oklahoma to western AR on Wed. the
models have better than average agreement and continuity for this
system, so the models are equally weighted for QPF and temperature
profiles.
The highest probabilities for 0.25 inches of icing are in
northeast TX to northwest LA and adjaceent southwest AR.

Petersen