A burst of warm weather made the Bay Area blisteringly hot Tuesday afternoon – especially in North Bay, East Bay and South Bay valleys – temperatures in some areas climbed past 100 degrees.
By 11 am, temperatures were already 15 to 20 degrees warmer than a day earlier. San Jose hit 101 by late afternoon as Oakland’s Piedmont Pines weather station, and Santa Rosa reached 105. Triple digits also hit Santa Cruz, and Scotts Valley and Felton in the Santa Cruz mountains; and the same was expected in Cloverdale, Livermore, Concord, Brentwood and Gilroy, according to Roger Gass, a meteorologist with the National Weather Service’s Bay Area office.
“We’re definitely on the warm-up,” he said.
Tuesday afternoon, he drove hordes to Ridgway Swim Center in Santa Rosa. Families and summer camp groups lined up at the door when its afternoon session opened, many worried they’d be turned away. All made it inside.
Aquatics supervisor Donald Hicks said Tuesday’s pool visitors weren’t seen since the pandemic – a welcome return.
“I’m spending extra time with my brand-new lifeguards,” Hicks said. “This is a lot for them.”
Poolside, watchful parents and grandparents took refuge under umbrellas and straw hats while kids swam and lined up for the waterslide.
“Just a typical summer Santa Rosa day,” said Ted Fullmer, who watched his 9-year-old grandson, who’s visiting from Idaho.
The weather service issued a heat advisory Tuesday from 10 am to 10 pm Older adults, people with fragile health .
Warm weather was also expected elsewhere around the bay – with 80s along the coast and upper 80s to lower 90s around the bay. Temperatures were expected to peak by 1 pm along the coast but not until 4 pm inland.
Some of the hottest temperatures expected in Concord, forecast to hit 104; Cloverdale and Brentwood at 103; and 101 in Livermore, Morgan Hill and Gilroy.
But the heat will begin to ease before it develops into a multiday heat wave, Gass said.
Cooler, damper onshore breezes were forecast to pick up in the afternoon and gradually work their way inland. Temperatures will drop by 10 to 15 degrees on Wednesday but remain above normal. Normal temperatures this time of year are in the mid- to upper 60s along the coast, 70s around the bay, lower 80s in the North Bay and upper 80s in the outer East Bay, Gass said.
Tuesday’s heat up is a bit of a one-off, a repeat of June 10 when Bay Area temperatures soared – but just for a day. And Bay Area residents seem to take it in stride, not rushing to hoard fans and air conditioners, or even ice cream cones.
Despite Tuesday’s predicted spike in temperatures, most cities and counties were not yet taking precautions, and stores were not reporting any dearth of fans and portable air conditioners. But Santa Clara County opened cooling centers in Milpitas, Mountain View, Santa Clara, San Jose and Sunnyvale Tuesday afternoon and said they may stay open through Friday if the heat persists. Locations and hours are listed at www.preparescc.org/hotweather along with any additional centers that are open.
At Stanford University, the power went out Tuesday afternoon when the heat increased demand for air conditioning. Power was out throughout the campus, possibly owing to the failure of one of the main Pacific Gas and Electric Co. transmission lines that supplies the campus, according to university officials.
Hardware stores in parts of the East Bay were prepared for triple-digit heat with fully stocked shelves of portable air conditioners and fans.
At Bill’s Ace Hardware in Concord, a worker said that there was a line out of the door for people buying portable air conditioner units and fans. But Wilma Hawkins, the store’s general manager, said she was not worried about selling out because “we have a child.” She said she expected more people to flood the store “as soon as it gets over 100 degrees.”
For Hawkins herself, she wasn’t worried about the heat.
“I’m a lizard, I like the heat,” she said laughing. “I’m happy for it.”
In Antioch at Ace Hardware, Selena Ortez, the manager, said she still has one air conditioning unit available in the store. But no one has yet to buy that or the fans or pools or the other “kinds of weather stuff.”
Ortez said she had a customer buy the AC unit on Monday “to prepare” for Tuesday’s heat. ” But that’s it, ”they said. “I’m not really getting a lot of customers for fans, but they might already be prepared.”
Concord senior citizens have not opened officially as a cooling center because “we only have a few days in the heat, but people are more welcome to come here. air conditioners until 4:30. ”
San Francisco homelessness officials said they were handing out water and keeping the eye out for people who might suffer the effects of the uncommon heat.
No multiple-day heat waves are forecast, Gass said.
But meteorologists warned of the slight possibility of dry thunderstorms in the Bay Area and along the Central Coast.
They were monitoring the flow of monsoonal moisture from the Gulf of Mexico, which could meet the heat and produce thunderstorms with lightning strikes that would pose a fire threat. Thunderstorms are considered possible on Wednesday and Thursday, according to the National Weather Service.
In Santa Rosa, by the Ridgway pool diving board, a group of kids treading water chanted a challenge: “Belly flop! Belly flop! ” egging divers on.
Most ignored them, but one boy seems enlivened by the dare. Camp counselor Brenda Cano couldn’t help but counter the cajoling.
“Don’t do it, that hurts!”
A boy took the challenge, and landed on the water with a slap.
He resurfaced smiling.
Michael Cabanatuan (he / him) and Sarah Ravani are San Francisco Chronicle staff writers. Email: mcabanatuan@sfchronicle.com and sravani@sfchronicle.com Twitter: @ctuan and @SarRavani