The storm, accompanied by high winds, thunder, lightning and hail the size of quarters, sent the new graduates of Absegami High School in Galloway and their families running for cover, WPVI-TV, Philadelphia, reported. It hit about an hour after the ceremony began at 6 p.m.
Diplomas had already been handed out, KYW-TV said.
No serious injuries were reported.
The ominous cloud on the horizon proved to be what is known as a wall or pedestal cloud, Cecily Tynan, the WPVI meteorologist, said. Unlike a funnel cloud, it does not rotate and will not be a tornado if it hits the ground.
Other parts of the area near Atlantic City were hit by bad weather late Thursday, including hail of up to an inch in diameter.