LOS ANGELES — A thunderstorm formed so rapidly over a Southern California beach that experts said Monday it was impossible for anyone to predict a lightning strike would turn a day of carefree fun into one of terror.

The phenomenon so rare that lifeguards lack an emergency warning system struck Sunday afternoon at Los Angeles’ popular Venice Beach, killing a 21-year-old man and injuring a dozen others.

Along the beach, famous internationally for its jugglers, skaters, medical marijuana dealers and boardwalk preachers and hucksters, panic instantly set in.

“All of a sudden, there was a huge explosion and everyone dropped to the ground. I thought, ‘Is there a bomb? Are there fireworks?’ The sky got black and then it started downpouring,” said Sam Solomon, a 24-year-old outdoor marketer from Los Angeles.

Although a commonplace in many other parts of the country, lightning rarely ever strikes the sand along the beaches of the Western U.S., climatologist Bill Patzert said. As a result, Southern Californians were completely unprepared.

“In Florida, under similar conditions, they might have asked people to clear the beaches. But not here,” said Patzert of NASA’s Jet Propulsion Laboratory.

Patzert said the storm that materialized over the beach did so rapidly and was so isolated that he couldn’t say anyone was to blame for not predicting it.

“It’s hard to find fault. I’d say impossible, actually,” he said. “It was a small, isolated system, and it just hit. It’s not as if it moved up the coast and kept repeating itself. It was tragic, but it was a one-shot deal.”