Whirlwind - the key word there is 'canyon'. If lightning strikes precisely at the far end of a canyon, the sound pressure wave bounces off everything below the top of the canyon creating a harmonic resonance in some cases that can actually be felt through the ground. Once the pressure wave reaches an open end to the canyon or the terrain just naturally flattens out, the pressure wave then releases it's energy as a regular thunderclap. This is a fairly common occurance west of the Mississippi River in many states and you can feel fortunate that you experienced one of these. Kind of frightening the first time it happens in your immediate area. The sound pressure wave was probably traveling at close to 1130 feet per second, so that must have been a fairly long canyon.
BOOM !
