Consider this your official reminder: Fall did arrive last Saturday.

But you might forget that this weekend when temperatures creep upward, creating some unseasonably warm weather throughout the Bay Area.

"If you're having a street fair in the East Bay or San Jose, you're going to be selling a lot of ice cream and beer," said Steve Anderson, a forecaster with the National Weather Service. "It's going to be nice, sunny and warm. But that's what people look forward to in the fall when you live here, especially on the coast."

While nowhere near records, daytime highs will rise to the mid-to-upper 80s in San Jose on the weekend, topping out at 88 degrees on Monday. In places like Concord and Livermore in the East Bay, temperatures will be about eight degrees above normal -- reaching the low 90s on the weekend and spiking at 95 on Monday, according to weather service forecasts.

But above-average temps in San Francisco should mean postcard weather with the highs reaching the mid-70s.

"It's some of the warmest weather in San Francisco that we've seen there since June," Anderson said. "It's going to be very nice."

That's good news for The Polk Street Blues Festival, which is hoping to draw between 30,000 and 40,000 visitors for the two-day event.

"San Francisco always has great weather when the rest of the Bay Area is sweltering," said Sarah Tipple, project manager for Steven Restivo Event Services, which is staging the festival. "You never know for sure with weather, but we do know that it's not going to be rainy, which is great for us."


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And it will be heating up throughout the region -- changing a pleasant weather pattern that has seen cooler-than-average temperatures this September. Jan Null, of Golden Gate Weather Services, said the average daytime highs have been six degrees below normal in San Francisco and three-and-a-half degrees lower in San Jose.

But now a high-pressure ridge is limiting the cooling, offshore breezes.

"Cut off the air conditioner, and it warms up," Null said. "It's as simple as that. But this is very normal. In fact it would be abnormal if we didn't have this kind of warm-up."

Anderson added that it's common during fall months to have three-day hot spells.

"This will be the first round of Indian summer-type weather that we've seen this year," Anderson said. "Then the cooler breezes kick up and the temperatures go back to normal."

But that probably won't happen until Tuesday.

So on Saturday afternoon, when Saratoga hosts the fifth-annual Heritage Day event, it could be uncomfortable for the more than 40 people who will be wearing historic garb dating back to the 1850s as they honor important people from the town's past.

"If you're in costume, it's a bad thing," said Annette Stransky, president of the Saratoga Historical Foundation, of the heat.

But Stransky was laughing, and she quickly added: "It's going to be a perfect day to be out and about. The weather will be great, and bring your camera because the costumes really are stunning."

Contact Mark Emmons at 408-920-5745.

Weekend forecast
The National Weather Service is forecasting sunny, unseasonably warm temperatures from Saturday through Monday. In the East Bay, daytime highs should range from 90 degrees on Saturday to 95 on Monday.
San Jose should hit 84 degrees Saturday, 86 on Sunday and 88 on Monday.
San Francisco will be in the low 70s on the weekend and reach 74 on Monday.
Santa Cruz will see 82 on Saturday and top out at 85 on Monday.