That is because such a storm acts like a train, with individual storm cells rapidly reforming behind each other in a line, following a narrow path as if on rails. (timesunion.com)
The severe turbulence found in storm cells alone should make the pilot of a small plane very wary. (scientificamerican.com)
1:45 pm EDT UPDATE: Looking at the radar there are a few isolated storm cells northwest and southwest of Pocono Raceway. (sbnation.com)