[11] At 7:00 a.m. EST (13:00 UTC) on April 20, observations from local stations indicated a low-pressure area of at most 29.6 inches of mercury (1,002 mb) over the Lower Mississippi River near Memphis, Tennessee.
A counterclockwise circulation was evident on the daily weather maps, with a trailing cold front approaching Mississippi from the west.
An influx of warm, moist air from the Gulf of Mexico yielded surface temperatures of 70° F or greater as far north as Nashville, with mid to upper 70s over the southern halves of Mississippi and Alabama.
[13] Due to a steep temperature gradient and low-level moisture, atmospheric instability was conducive for convective growth, with a convergence zone providing sufficient wind shear for supercells.
[13] Despite widespread cloud cover, high temperatures in Alabama attained 85 °F or higher at several stations, illustrating the richness of the unstable air mass.
In Marion County alone, the 1⁄2-mile-wide (0.80 km) tornado killed 20 people and injured about 200, leveling 87 homes and damaging 100, especially in the Hackleburg area.
There, four people died, many homes were destroyed, and numerous cattle were killed; notably, a Ford vehicle, hurled some distance, was stripped of its wheel casings.