SPC AC 201724
Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1224 PM CDT Wed Oct 20 2021
Valid 211200Z - 221200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE OHIO/TENNESSEE VALLEYS INTO THE CENTRAL/SOUTHERN
APPALACHIANS...AND WEST TEXAS...
Showers and storms with isolated strong to damaging wind gusts will
be possible Thursday across parts of the Ohio/Tennessee Valleys into
the central/southern Appalachians. Isolated storms capable of
marginally severe hail and strong gusts may also occur over a small
portion of west Texas.
...Ohio/Tennessee Valleys into the Central/Southern Appalachians...
An upper trough/low should move quickly eastward across the Great
Lakes and Midwest through the day on Thursday. An attendant cold
front will sweep eastward across these regions, much of the OH/TN
Valleys, and into the southern/central Appalachians by Thursday
evening. Limited low-level moisture, with surface dewpoints
generally in the mid 50s to low 60s, is forecast to return northward
ahead of the front. Modest diurnal heating, poor lapse rates, and
cloud cover will likely limit how much boundary-layer instability
can develop Thursday afternoon, particularly across the OH Valley
and central Appalachians where low-level moisture will be less.
Even so, low-topped showers and storms should develop by early
Thursday afternoon along much of the length of the cold front. The
weak forecast instability coupled with fairly strong
west-southwesterly winds at low/mid levels should support some
threat for isolated strong to damaging wind gusts as storms spread
eastward through the day. Slightly greater instability may be
realized from TN southward into AL/GA, but deep-layer shear should
be weaker across this area than in the OH Valley. This isolated wind
threat should quickly wane early Thursday evening as convection
encounters the higher terrain of the Appalachians and a less
favorable thermodynamic environment.
At least isolated storms should develop Thursday afternoon over the
higher terrain of the Davis and surrounding mountains in west TX as
a weak mid-level disturbance moves over this region. Although
low/mid-level winds are forecast to remain fairly modest, there will
be some veering of the flow with height, and steep low/mid-level
lapse rates should also be present. Any storms that can form in this
regime may pose an isolated hail and strong/gusty wind threat
through early Thursday evening.
Most 12Z guidance indicates the potential for convection across
southern OK will be rather limited Thursday night into early Friday
morning. While a storm or two may still develop, weak instability,
marginal deep-layer shear, and meager large-scale ascent all suggest
the threat for anything more than small hail should remain low.
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