Heat advisory issued as Bay Area sees major temperature increase

A building ridge of high pressure in the Pacific Ocean is sending temperatures soaring in the Bay Area starting Wednesday, with even hotter weather expected Thursday.

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Lamont Bain, a meteorologist with the National Weather Service, explained the “culprit” for the rapidly rising temperatures is “offshore flow,” where the wind blows from inland out to the ocean, bringing hot weather to normally cooler coastal regions.

“When we see these offshore flow patterns, that really allows us to bake, because we don’t get the influence of the marine layer,” he told SFGATE.

On Wednesday, temperatures in much of the Bay Area will be 5 to 15 degrees above average, while Thursday temperatures are more likely to run closer to 20 degrees above normal, Bain explained. The North Bay, where red flag warnings have been issued, could see temperatures 25 degrees above normal.

The National Weather Service issued a heat advisory starting noon on Thursday for parts of the Bay Area. The advisory comes as temperatures are expected to reach 103 for the interior valleys, especially at high elevations. Even coastal areas could see temperatures reach between 80 and 96 degrees.

Temperatures inland in the East Bay are already climbing, with Concord expected to reach 96 on Wednesday and 104 on Thursday. San Jose is expected to near 90 on Wednesday and about 95 on Thursday. And while cooler, downtown San Francisco is expected to see a major temperature shift, rising to 77 degrees on Wednesday and 84 on Thursday.

Bain said the hot, windy weather will also dry out fuels in the hills, increasing the risk of wildfires. “Even just a couple of days of sunshine will dry those grasses out,” he said.

The weather service issued a red flag warning for parts of the North Bay interior mountains and the East Bay Hills starting Wednesday evening at 11 p.m. and lasting until 9 a.m. Thursday. The warning is in response to a forecast of low humidity and high winds of 10 to 20 mph, with gusts expected to top out at 45 mph or up to 60 mph on the ridgetops. 

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It’s the first red flag warning of the year for the Bay Area and comes as Cal Fire is still working to contain a fire in nearby Yolo County.

“The combination of the hot, dry, and windy conditions will result in critical fire weather conditions where any fires that start could take hold and spread rapidly,” the weather service forecast discussion read. 

PG&E said it plans public safety power shutoffs for customers in part of Colusa, Glenn, Lake, Sutter, Tehama and Yolo counties starting Wednesday evening. Bay Area counties of Sonoma, Solano and Napa are also “likely” to see power shutoffs because of the high wind.

On Friday, the temperature is expected to drop a bit, Bain said, although models are unclear by how much, especially if winds coming from the ocean are “weaker” than expected.

While the weather is expected to be sunny and dry this week, the weather service is reminding Bay Area residents that flooding could be an issue due to “high astronomical tides.” The tidal surge will likely be about 1.2 feet above normal on Thursday and about 1.5 feet above normal on Friday, leading to flooding in low-lying areas near the bayshore.