This is a phenomenon known locally as “second summer,” said Dylan Flynn, a meteorologist at the National Weather Service in California, who noted that early fall was generally when San Francisco sees its highest temperatures. “But this one is more extreme,” Mr. Flynn said in a phone interview.

SAN FRANCISCO - San Francisco experienced its hottest day of the year on Tuesday, with temperatures hitting the mid-90s. Residents flocked to local beaches to escape the heat. Berkeley's Zach Pine said the San Francisco weather was similar to Southern California's weather.