Lightning can strike more than ten (10) miles away from the center of a thunderstorm, beyond the audible range of thunder. Therefore, if you hear thunder, you are within striking distance of the storm.
Seek shelter if the “flash-to-bang” delay (length of time in seconds from the sight of the lightning flash to the arrival of its subsequent thunder), is 30 seconds or less, and that they remain under cover for 30 minutes after the final clap of thunder.
A 30 second lead time is necessary prior to the storm’s arrival because lightning often strikes ahead of the storm center. A 30 minute wait after the last thunder is heard is necessary because the trailing storm clouds still carry a lingering charge. This charge can and does occasionally produce lighting on the back edge of storm, several minutes after the rain has ended.