I was flying a Pan American A-300B4 (Airbus) from New York to Mexico City. Our route took us directly over Biloxi, Mississippi and then over the Gulf of Mexico. This took us over the Gulf just south of New Orleans. As we passed by Biloxi I picked up a very strong radar return about 100 miles in front of our plane directly in our path. It appeared to be a very small but powerful thunderstorm so I didn't pay much attention to it. As we approached the radar return New Orleans Air Traffic Control Center called to ask we could see "something" directly in front of us. I replied we had a small but strong thunderstorm and asked to deviate to the left of the return, still assuming it was a compact but powerful storm.

We made our turn to pass just to the left of the return.  As we got within a half-mile of the radar return it completely disappeared.  It was there during one sweep of the radar and gone the next.  The lights from New Orleans were very bright and I could see clearly towards the city.  There weren't any clouds at all, much less a thunderstorm as we flew by the spot where the return had been.
 Unfortunately, I was distracted at that time and didn't give any more thought to what had happened, although it was very strange.
 It was a few years later that I suddenly thought about what had happened and that it may have some significance.  I would like to have contacted New Orelans ATC to see if the controller that was on duty that night was still available but too much time had passed.
 This is my 2nd and final report of "sightings" during 45 years and 20,000+ hours flying.