National Weather Service forecasters said a line of thunderstorms moved into Long Beach, the South Bay and Catalina Island around 4:30 p.m.
Los Angeles County lifeguards cleared Redondo Beach after witnessing lightning strikes shortly before 5 p.m. Hermosa Beach was then closed, then lifeguards shuttered all beaches in the county.
The public was urged to stay off the beaches and out of the water, and remain indoors until the weather clears.
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City News Service contributed to this report.