San Francisco, where even the postcards feature fog, rarely sees temperatures over 90 degrees. Even more rarely does the coastal California city see such temperatures in October.

But on Tuesday, San Francisco hit 94 degrees.

The heat has prompted officials to issue an excessive heat warning through Thursday and to encourage residents to go to cooling centers, public pools and to hop on the ferry, which was offering free rides on Wednesday. In some inland areas, temperatures are soaring above 100 degrees.

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.

“It’s very rare to be in the 90s at all in San Francisco. But in October, it’s even more rare,” he added. “The last time we had 94 all the way in October was 2012.”

The heat was expected to peak mid-week, from Tuesday to Thursday, but high temperatures were expected to last through the weekend, said Mr. Flynn, who described the heat wave as the sort of “long duration” event that can raise the risk of illness, or even cause heat-related deaths.