Overshooting tops develop on thunderstorms that have particularly strong updrafts.
Overshooting tops typically develop on Cumulonimbus capillatus incus clouds. The updraft punches through the equilibrium level (and thus the anvil), rising higher due to its momentum, despite not being buoyant anymore. An overshooting top is usually short-lived, several tens of seconds, up to several minutes. In rare cases, it may persist much longer.
An overshooting top, particularly a persistent one, is an indication of a potentially severe thunderstorm.
An overshooting top is difficult to see, and can only be seen on distant thunderstorms. It is readily apparent also in satellite imagery, in particular at low Sun angles (i.e. morning and, more often, late afternoon).