Higher temperatures were concentrated in the fringes of the Bay Area, while most urban and coastal areas stayed cooler, according to a statement from Logan Johnson, a National Weather Service meteorologist.
The Angwin area in Napa County recorded the Bay Area's highest temperature today with a high of 106, and Calistoga and Napa also exceeded 100 degrees, according to a weather service report.
In Alameda County, Dublin hit 103 and Livermore and Pleasanton reached 100, while in Contra Costa County Walnut Creek reached 101 and San Ramon, Danville and Concord all reached 100.
In the South Bay, only Gilroy reached 100, but most other cities were in the 90s.
Johnson said temperatures were expected to climb a few degrees on Saturday.
"It should be a few degrees warmer Saturday, so perhaps up to 110 degrees in the far interior of Napa County and the East Bay," Johnson said.
County and city health officials in Napa, Contra Costa and Alameda counties said they did not plan to open cooling centers, in part because the heat wave is predicted to be short-lived.
"The temperatures are expected to drop into the 50s overnight, and the biggest indictor of risk to health is when you have that sustained high heat so that houses do not get a chance to cool off," said Napa County Public Health Officer Dr. Karen Smith.
Contra Costa County health officials have posted tips for handling hot weather at cchealth.org/heat. Residents are being encouraged to visit air-conditioned public settings such as libraries, shopping centers, senior centers and movie theaters if they do not have air conditioning at home.
Temperatures should drop by Sunday, and there is even a slight chance of isolated showers or thunderstorms, especially over the Sierras, according to the weather service.
Temperatures greater than 110 degrees are also being forecast for this weekend in parts of Southern California including the Inland Empire, Central Valley and southern deserts, prompting state Department of Industrial Relations officials to issue a warning for workers operating outdoors.