Surrounded by dozens of people stretched out on the grass under the sun outside the Jewish Contemporary Museum in San Francisco, Michelle Sintov opted for the shade.

Not many followed her lead, as most tourists and San Franciscans left their layers behind Tuesday to enjoy what forecasters predicted will be the warmest day of the week before a 20-degree drop in Bay Area temperatures by the weekend.

Sipping slowly on her soup, the 46-year-old city resident said she had recently gotten a lot of sun as justification to sit underneath a row of trees.

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“It is warmer than the normal day,” Sintov noted. “It’s nice to come outside.”

San Francisco remained relatively cooler than most cities in the Bay Area, particularly along the coast with temperatures in the 60s and downtown getting into the low 70s, said Roger Gass, a meteorologist with the National Weather Service in Monterey.

In Oakland, temperatures reached into the upper 70s as a massive collection of Warriors fans lined the streets to celebrate the back-to-back NBA champions.

San Jose reached a balmy 85 degrees and, by Tuesday afternoon, Concord was the hottest city in the Bay Area at 92 degrees, according to the National Weather Service.

“It’s definitely going to be a noticeable warm-up, (but) it’s not anything completely out of the ordinary,” Gass said Tuesday morning.

However, that doesn’t mean other areas in Northern California avoided sweltering temps.

In Napa Valley, temperatures were just a few degrees shy of triple digits, Gass said. Officials are preparing for fire season, but he noted that shouldn’t be a concern this week.

“When we have hot and dry conditions and strong offshore winds, that could sometimes rapidly spread fires,” Gass said. “We’re not having that wind component.”

The same scorching weather was anticipated in Brentwood, Antioch and Pittsburg, Gass said, but “we are not expecting any widespread records to be broken.”

On Wednesday and Thursday, the forecast is a few degrees cooler throughout the region before a significant cold front comes to the area Friday and Saturday.

“A lot of spots will cool almost 20 degrees compared to what we are anticipating today,” Gass said.

In San Francisco, many didn’t seem fazed by the impending cooldown, including Sarah Knapp, who stopped in the city to enjoy the sunshine before continuing her road trip down the coast that afternoon.

Knapp, a San Diego resident, kicked off her yellow sandals as she turned the page of her book and burrowed her feet into the thick grass.

“It’s really beautiful, so beautiful,” she said, squinting into the sunlight.

Sarah Ravani is a San Francisco Chronicle staff writer. Email: Twitter: @SarRavani