Cooler weather may bring Bay Area a bit of rain by Sunday

The natural air conditioning that always has been one of the Bay Area’s main selling points will continue to cool off the region through the weekend, and National Weather Service forecasters believe it may also be mixed with a rare late-summer spate of rain.

There just won’t be much of it.

“The best chances of it are in the North Bay,” meteorologist David King said. “Even then, it won’t be a substantial rain. It might wet the sidewalks.”

The cool air is coming from the normal return of a deeper marine layer and the formation of stratus clouds 1,000 feet above the surface, King said. The result is that the fog is thicker and lasts longer, and the sun’s heat has a thicker area to penetrate.

Temperatures were not expected to get past 84 degrees anywhere in the Bay Area on Thursday, with the inland region again being the warmest place to be. San Jose was expected to get to 75, Oakland to 67, San Francisco to 63 and Monterey to 62.

The rain is expected to arrive Saturday night or Sunday morning, when the cold upper-level trough pushing down from the Bay of Alaska reaches the northern part of the state. King said some elevated areas in the North Bay might see one-tenth of an inch of rain, although some pockets of the storm could dump as much as a quarter-inch.

Some rain may fall in Alameda and Contra Costa Counties, as well as in the South Bay, King said.

“It’s what we call a nice helpful rain,” King said. “But it’s nothing that’s going to allow us to forget about fire season or anything that’s gonna drench us.”

It also won’t stick around a long time, making way for a brief warmup: By Monday, clear skies and temperatures in upper 80s and low 90s are expected. Patchy clouds are anticipated to return to the sky on Tuesday.