Discussion from the SPC:
…SUMMARY…
Severe thunderstorms capable of tornadoes and extensive wind damage are possible across parts of Louisiana, Mississippi and Alabama on Thursday. Scattered severe storms are also possible as far east as the Florida Panhandle, and as far north as western Tennessee.…Synopsis…
An upper trough will amplify as it approaches the lower MS Valley with mid to upper winds increasing to 60-80 kts. At the surface, a cold front/outflow composite boundary will extend from eastern TX into AR in the morning, and will shift east through the period. Ahead of this early activity will be a deepening surface trough. Preceding the front and early storms, southwesterly low-level winds will create a surge of theta-e across LA, MS, and AL during the day, with mean boundary layer dewpoints increasing to 68-70 F.Very strong large-scale lift will develop over the lower MS Valley during the day, with intense deep-layer differential divergence. Cooling aloft will also accompany the cold front, further aiding destabilization. The result will be widespread thunderstorms, with corridors of damaging wind and tornadoes.
….LA…MS…AL…Western FL Panhandle…
Storms are expected to be ongoing Thursday morning along the cold front/outflow, from southern AR into eastern TX. Some of these storms could produce strong wind gusts or marginal hail.Later in the day, a strong surge of deeper low-level moisture will spread across LA and MS, coincident with 850 mb winds increasing to over 50 kt. Both instability and shear will increase, and conditions will become favorable for supercells and tornadoes. Forecast soundings indicate effective SRH on the order of 300-400 m2/s2 across the entire area. Thus these midday storms may be severe with a few tornadoes possible should they remain cellular.
Meanwhile, the activity along the cold front is expected to intensify as it encounters this stronger instability into central and eastern LA, and continuing across central and eastern MS during the day. By late afternoon, an intense QLCS is forecast, likely absorbing the earlier activity and producing corridors of destructive winds and tornadoes. The most intense activity may also spread into western Alabama, with a gradual decrease in overall intensity overnight across eastern AL and into FL.
Early convection due to warm advection in the 15-18Z period lends some uncertainty as to air mass quality for latter-day convection with the front. If the rain is too widespread across LA and southern MS, it could temper instability for the expected QLCS. On the other hand, if the midday activity remains more cellular, several tornadoes could occur. As such, will defer possible upgrade to Moderate Risk to upcoming Day 1 updates.
