Last evening there were some pretty severe looking radar tracks in Pensacola. It was getting dark here in the Navarre area and a lightning flash caught my attention and I began to count. When the rumbling started I estimated the distance at 9 miles. A ground shock wave had arrived which shook the house causing the dishes in the cupboard to rattle. The rumbling went on and on. I was quite surprised in the duration. I thought perhaps a plane had crashed or a fuel truck had exploded on highway 98 so began surfing the tv channels for news of the expected event, zip. Another flash, the rumbling began after about 21 seconds. At the beginning of the rumbling, I started the count, it stopped after approximately 31 seconds.
I am trying to account for the reasons for such a long period of sound waves. It could be reflections of sound bouncing back and forth, but the amplitude and frequency did not seem to be too variable. The other thought was multiple lightning strikes with flashes not visible to me from my location.
Last week there was a lightning storm which produced strikes very close to home. Instead of the low frequency flutter there was a long duration of crackling which had an excessively long duration.