The crew of the Airbus had been keeping close tabs on the weather in the MIA area, including the report of a tornado in the airport area. However, due to the storm's height and intensity, the turbulence it encountered may have been produced by some sort of convective-induced wave or rotor and the true cause may never be known for sure. Sometimes even well-respected rules of thumb can be inadequate when it comes to thunderstorms. A conference paper titled Thunderstorm Induced Gravity Waves as a Potential Hazard to Commercial Aircraft just begged for a look. Although it sounds like science fiction, the term refers to a newly identified form of heavy weather.
Gravity waves are a phenomenon associated with thunderstorm development.
The atmosphere acts to constrain the vertical dispersion of wave energy and thus produces a horizontal wave-guide. . . . Cool thunderstorm downdrafts help to increase the stability of the boundary layer and thus increases the likelihood of gravity wave development. The resulting waves are symmetrical with uniform wavelengths and wave speeds. Numerous gravity wave events have been observed visually and on radar by researchers in the Memphis and Dallas airport areas since 1997. Although waves have been identified throughout the year, the peak activity occurs, as one would expect, during the late summer peak of thunderstorm activity.