How hot will it get?

Tuesday marked the start of a warm spell in the San Francisco Bay Area that’s forecast to peak Thursday and linger into Friday and Saturday, forecasters said.

The next few days are expected to bring the sort of gloriously sunny, mostly fog-free weather to San Francisco, which can be socked in with fog in July and August, that locals anticipate as summer wanes and fall begins. Meanwhile, inland valleys, which sometimes see temperatures soar into the 100s in summer warmups, are predicted to get only into the 90s, the National Weather Service said.

The rise in temperatures is expected to bring a significant and noticeable warmup to the region — but this is not a heat wave, according to weather service meteorologist Sean Miller. Heat waves occur when temperatures soar well above their historical averages across the region for two or more days, and this is not an event where well above-normal temperatures are forecast to be widespread. 

“By and large for our whole area, it’s not going to be super hot compared to some of the events we had earlier in the fall, especially for inland areas,” Miller said. “The key thing with this one is the places at the coast that missed out on the heat last time are going to get it this time. If it plays out the way it should, this will be one of the warmest days of the season for the coastal spots.”

Coastal areas that have recorded afternoon highs in the 60s in recent days will see temperatures in the 70s and 80s, according to the weather service, while locations along the bay front and inland areas that have been in the 70s and 80s will inch up into the 90s. Light northeasterly winds, also known as offshore winds, will help push warm air from inland valleys toward the coast, the agency said. 

When warm weather strikes the Bay Area, inland areas often see extreme temperatures into the high 90s and low 100s while the coast remains cool. This is a common scenario in summer, when the fog hugs the coast and the temperature difference between, for example, San Francisco’s Ocean Beach and Walnut Creek in the East Bay can be over 20 degrees. In this current warmup, the difference in temperatures between inland areas and the coast will likely only be about 10 to 15 degrees. This sort of setup is common in fall, when the fog tends to be lighter and the ocean breeze slackens, allowing coastal areas to warm up. 

The warm weather comes as a large ridge of high pressure that has been over the Pacific Ocean moves inland over the western states. The system will likely be centered over the Pacific Northwest but spread all the way across California. “All the Western states are mostly under a ridge of high pressure for a couple days,” Miller said.

Temperatures Tuesday climbed into the 60s and 70s at the coast and into the 80s inland. Wednesday is expected to bring a bigger warmup, with afternoon highs 5 to 10 degrees warmer Wednesday than they were Tuesday. Thursday will be a little warmer than Wednesday, and Friday will be only a tad cooler than Thursday, the weather service said.

The weather service gauge in downtown San Francisco is expected to hit a high of 82 on Wednesday, 85 on both Thursday and Friday, and 79 on Saturday, according to the weather service. Miller said other locations in the city could hit 90 degrees. Whether the downtown gauge reaches 90 degrees depends on when the ocean breeze picks up in the evening. “I would say early-to-mid afternoon would be our best shot at 90, because by the time you get into the late afternoon, the sea breeze will kick up,” Miller said. “Basically, we’re going to have a battle of how quickly the temperatures can heat up with the light offshore winds that are going to happen through the day and before the winds turn onshore later in the afternoon.”

Across the bay from San Francisco, Oakland is expected to be only a few degrees warmer than SF in coming days, with forecast highs of 86 on Wednesday, 87 on Thursday, 85 on Friday and 84 on Saturday. 

In the East Bay, Walnut Creek is poised to reach 90 on Wednesday, 94 on Thursday, 95 on Friday and 94 on Saturday. To the north, Santa Rosa is forecast to record 89 on Wednesday, 91 on Thursday, 90 on Friday and 89 on Saturday. 

On Saturday, the coastal breeze is expected to become stronger and coastal locations are unlikely to break into the 80s, and inland areas will dip into the high 80s to low 90s, the weather service said.