Heat record broken in SF, but cool weekend on tap

Angelique So seeks relief from the heat in the shade of a tree at Heather Farm Park in Walnut Creek, Calif. on Wednesday, May 3, 2017. The Bay Area experienced one more day of unseasonably warm temperatures but a cooling trend is one the way. less Angelique So seeks relief from the heat in the shade of a tree at Heather Farm Park in Walnut Creek, Calif. on Wednesday, May 3, 2017. The Bay Area experienced one more day of unseasonably warm temperatures but ... more Photo: Paul Chinn, The Chronicle Buy photo Photo: Paul Chinn, The Chronicle Image 1 of / 90 Caption Close Heat record broken in SF, but cool weekend on tap 1 / 90 Back to Gallery

As the temperature in San Francisco soared to a record for the day Wednesday, climbing past the previous mark set in 2013, forecasters warned that the summerlike conditions will plunge nearly 20 degrees in some parts of the region starting Thursday.

At 82 degrees, it was the hottest May 3 in San Francisco since record keeping began in 1848, according to the National Weather Service.

The record-breaking heat in the city came a day after Richmond and Napa broke temperature records for the day by 1 degree. Richmond reached a high of 87 degrees, breaking a record set in 2014. In Napa, temperatures reached a high of 94 degrees, topping a mark set for the day in 1966.

Wednesday was the last day of the hot spell in the region before a “rolling cooldown” goes into effect, and San Francisco and other parts of the Bay Area start to shift into normal May weather, said Scott Rowe, a forecaster with the National Weather Service.

“Things are going to be close to record highs for many locations,” Rowe said just hours before the temperature rose to a record-breaking high in San Francisco on Wednesday.

The Bay Area Air Quality Management District issued its first Spare the Air alert for smog in 2017, deeming Wednesday’s air quality to be “unhealthy.”

“Although it is still spring, hot summerlike temperatures combined with exhaust from traffic on Bay Area roads will cause smog and hazy skies,” said Jack Broadbent, the air district executive officer. “Climate change causes more extreme temperatures and is closely linked to air quality.”

Commuters were advised to take public transit, carpool or work from home to help the air quality.

A dramatic cooldown in San Francisco begins Thursday when the forecast drops nearly 10 degrees, with highs expected to peak in the upper 60s. San Franciscans can expect another 5- to 10-degree decrease for Friday and weekend temperatures will be in the upper 50s to mid 60s.

“We are looking at the chance of low clouds and fog to return,” Rowe said of the weekend forecast. “It’s the season.”

In the East Bay, the forecast is similar to San Francisco with temperatures decreasing by 10 degrees Thursday from the 80s and 90s recorded in some cities Wednesday.

Elsewhere in the East Bay, cooler weather will start to set in with a high in the low 70s on Thursday. The weather was expected to cool off even more Friday and into the weekend with highs in the mid 60s.

Temperatures in San Jose will plummet close to 20 degrees from Wednesday into the weekend, Rowe said.

San Jose hit a high of 92 degrees Wednesday, but temperatures in the city are expected to fall to the upper 70s on Thursday. Temperatures are forecast to drop even more Friday and Saturday with a forecast in the upper 60s, Rowe said.

As temperatures begin to cool down, Rowe warned that winds would be picking up for the weekend.

An ocean breeze of 20 to 30 mph is expected to sweep through San Francisco and into the rest of the Bay Area starting Saturday.

Gusts could reach 50 mph in higher elevated areas, including the San Mateo mountains, Rowe said.

Sarah Ravani is a San Francisco Chronicle staff writer. Email: sravani@sfchronicle.com Twitter: @SarRavani