Ordinary thunderstorms have the capacity to transform themselves into multi-cell storms of ever growing complexity. Such multi-cell storms display a marked increase in severity and their lifetime can be extended by a factor of ten or more. The grandfather of all thunderstorms, however, is the super-cell thunderstorm. It should be noted that even ordinary thunderstorms are capable of mutating into super-cell storms. These storms subside very slowly.
--Storm Dynamics and Hail Cascades
by L. D. Sax and W. S. Dutton