On-shore flow set to bring an end to Bay Area’s brief heat wave

As warm as it already was in the Bay Area when the sun arose Thursday — temperatures in the far inland areas of the East Bay already were in the mid-70s by 7 a.m., and they already had cracked 70 in Oakland, San Mateo and other coastal locations — another element was beginning to become more influential.

“We’re finally getting some on-shore flow,” National Weather Service meteorologist Nicole Sarment said of the breeze and cooler air that flows in from the Pacific Ocean during the region’s usual weather patterns. “That’s going help it not be quite as hot as it was on Wednesday. That said, the air mass is already warm.”

As a result, Sarment said it will be plenty toasty around the region for one more day. The region’s hottest places are expected to peak at 97-98 degrees, while cities along the water are expected to get to 80 degrees or close to it. A heat advisory for the region that started Tuesday was set to remain in effect until 10 p.m. Thursday, and temperatures were likely to be similar to what they read on Tuesday, when this three-day heat wave began.

That means they are likely to be 5-8 degrees cooler than Wednesday, when the thermometer set daily records in several places. Those spots included downtown Oakland, where the high of 97 topped the previous mark of 96 set in 1951; San Francisco International Airport, where the high of 94 topped the previous record of 92 set in 1951; and in Half Moon Bay, where the high of 83 topped the previous record of 79 set in 1979.

Unofficial readings showed the thermometer surpassing 90 degrees in areas of San Francisco. One unofficial reading shared by the weather service showed a 100-degree temperature in one San Francisco neighborhood.

The increased presence of the on-shore flow of cooler air is set to put an end to those extremes on Friday.

“The bigger thing is that the (high-pressure) ridge is going to get squashed down,” Sarment said. “That should make the onshore flow more normal. That’s going to lead to the return of the marine layer tonight.”

The marine layer — the region’s natural air conditioning — will accompany temperatures Friday that are expected are expected to peak in the low 80s in the hottest spots. Areas along the water are expected to stay in the 60s.

“There’s low pressure coming down from British Columbia,” Sarment said.

It’s also expected to help clean the air. The Bay Area Air District forecast air in the Santa Clara Valley to be unhealthy for sensitive groups with breathing issues Thursday. The air in the rest of the district is expected to be moderately clean. A Spare the Air alert in place on Wednesday expired, and the district did not issue one for Thursday.

The low pressure is not expected to usher in stormy weather, but it will oversee a long stretch in which temperatures are expected to remain quite mild. By the weekend, the weather service said the high temperatures in the hottest spots are likely to stay below 80.

“The temperatures are going to be moderate for a while,” Sarment said. “That’s the long-range forecast.”