Probability of damaging thunderstorm winds or wind gusts of 50 knots or higher within 25 miles of a point. Hatched Area: 10% of greater probability of wind gusts 65 knots or greater within 25 miles of a point.
SPC AC 171606
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1106 AM CDT Wed Mar 17 2021
Valid 171630Z - 181200Z
...THERE IS A HIGH RISK OF SEVERE THUNDERSTORMS WEST-CENTRAL
ALABAMA...NORTHERN AND CENTRAL MISSISSIPPI...NORTHEAST
LOUISIANA...FAR SOUTHEAST ARKANSAS...
...SUMMARY...
A significant tornado outbreak is expected with numerous strong and
a few long-track, potentially violent tornadoes. An initial round is
expected to begin across eastern Mississippi and Alabama this
afternoon. A separate swath should emanate from southeast Arkansas
to Louisiana this afternoon and spread east across Mississippi and
Alabama this evening into tonight.
...Deep South...
Minor change made to the categorical HIGH risk area to expand a bit.
Main change is to the underlying probabilities with the addition of
a 45 tor and 45 wind. A dangerous, long-duration tornado outbreak
expected to commence this afternoon and persist well into tonight
with multiple rounds of heightened tornado potential.
Ongoing convection is within an arc with a pre-frontal squall from
far east TX to northeast AR and then in a more west/east-orientation
from northeast AR to the AL/TN border area. The northern activity
will tend to reinforce the surface warm front approaching the TN
border with MS/AL this afternoon. The southwest band will likely
persist east and should breakup into a broken band of supercells at
some point this afternoon as low-level shear further strengthens
across the Ark-La-Miss region. Farther east, warm-advection showers
in southeast MS to west-central AL will likely deepen by early
afternoon into a separate swath of several discrete supercells in an
environment of strengthening low to deep-layer shear. Between these
two areas, the destabilizing warm sector with warming boundary-layer
temperatures and mid 60s to low 70s dew points, will support an
expansive plume of 1500-2500 J/kg MLCAPE.
By late afternoon and into early tonight, a low-level jet segment
will strengthen to at least 50-60 kt across MS/AL as the midlevel
trough approaches from the west, contributing to very strong
low-level shear (0-1 km SRH in excess of 300 m2/s2). Buoyancy will
be slow to decrease after sunset and with eastward extent based on
the prevalence of rather rich boundary-layer dew points, Very
favorable wind profiles (with 700-mb winds reaching 70-80 kt) will
maintain the threat for long-track, intense tornadoes with both
warm-sector supercells, as well as supercells within the broken band
near and ahead of the surface wind shift progressing east across MS
by early tonight. The strong tornado and significant damaging wind
threat while becoming more spatially confined with time, will likely
persist across parts of AL overnight and should spread into western
GA before 12Z.
..Grams/Bentley.. 03/17/2021
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NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 2000Z
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