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.Middle and lower MS Valley eastward into the TN and OH Valleys...

Complex forecast for Thursday with considerable uncertainty
regarding specific details of the overall forecast scenario owing in
part to prior thunderstorm activity on Wednesday and Wednesday night
through early Thursday. Some model solutions exhibit favorable
buoyancy/shear for the possible development of significant severe
thunderstorms during peak heating while other solutions suggest a
more mesoscale-dependent severe risk to develop within the area
generally defined within the 30% severe probabilities (category 3
risk). Early-day clouds/thunderstorm activity will likely modulate
both low-level moisture and 700-500mb lapse rates across the MS
Valley. This is reflected in the variability of the most recent
model guidance with the NAM implying intense storms redeveloping
across the northern half of MS northward into the lower OH Valley.
This is in contrast to the latest GFS and ECMWF models showing
widespread thunderstorms in a band during the morning near the MS
River moving east with little recovery/destabilization expected in
its wake. Yet, some cooling aloft in association with the
approaching mid-level low/cold pocket will contribute to
destabilization, in addition to surface heating during the day in
wake of early-day clouds/thunderstorms. A cold front is forecast to
slowly progress across the MS Valley and serve as a focus for
renewed thunderstorm development, perhaps as early as midday. As
previously described, considerable variability in the magnitude of
buoyancy is depicted in model guidance. Nonetheless, upwards of
500-1500 J/kg is forecast within an area of strong cyclonically
curved 500-mb flow. Forecast soundings show strong effective shear
(40-60 kt) which will likely result in storm organization and an
attendant risk for wind damage. The strength of the low-level shear
will conditionally augment a tornado risk for any mature supercells
or mesovortices in a squall line. Hail will be possible both near
the cold pocket with strong updrafts and in areas where mid-level
lapse rates are less convectively contaminated. The broken bands of
storms will move east in eastern parts of KY/TN and the eastern
portions of the central Gulf Coast during the evening and overnight
with a gradual weakening in storm intensity and expected severe
coverage.