SACRAMENTO, Calif. (AP) — Much of California was humid Tuesday, and the situation was expected to only get worse during the July 4 holiday week in parts of the United States, with nearly 90 million people under heat warnings.

According to the National Weather Service, the extreme weather conditions were caused by a high pressure area near the West Coast and a separate area that issued heat warnings and advisories from Kansas and Missouri to the Gulf Coast states.

California’s capital, Sacramento, was under an extreme heat warning expected to last until Sunday evening, with temperatures expected to range between 40.5 and 46.1 degrees Celsius.

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John Mendoza, 35, called it a “firehose of heat” as he walked around the Capitol Tuesday with an iced coffee in his hand. By 9 a.m., he had already been in a pool once — and he planned to go back later in the day.

“I felt like I had to stay underwater,” he said.

About 70 miles (113 kilometers) north of Sacramento, crews working in scorching conditions battled a wildfire in Butte County that forced the evacuation of about 13,000 people in and around Oroville. The blaze, dubbed the Thompson Fire, broke out before noon and sent a massive smoke plume that quickly grew to more than 3 square miles (7.7 square kilometers) by evening without any containment.

Firefighters lined the roads trying to prevent the flames from reaching homes, while helicopters dumped water on the rapidly spreading fire.

The governor’s office announced late Tuesday night that federal funding had been approved to help with firefighting efforts. Earlier this week, Gov. Gavin Newsom activated the State Operations Center to coordinate California’s response, dispatch mutual aid and support local communities as they respond to threats from wildfires and extreme heat.

As temperatures soared in Sacramento, Katherine Powers sought refuge in the shade of Cathedral Square. Powers, who is homeless, sipped sparkling water as she rested her bare feet on the shaded sidewalk.

Powers said she lent her shoes to a friend. She had not yet visited any of Sacramento County’s nine “cooling centers,” she said, because of the difficulty of carrying all of her belongings.

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“I just go to a park with a water fountain to stay cool, stay in the shade and just keep pouring water on myself, basically,” she said. “There’s not much I can do.”

Darlene Crumedy, who lives in Fairfield, says she doesn’t use air conditioning because it’s too expensive.

“I’m fine, I have a hundred fans,” she said, adding that she tries to stay indoors and drink cold water.

An analysis by The Associated Press found that heat killed more than 2,300 people in the United States last year, a record. That figure is likely a gross underestimate, dozens of experts told AP reporters.

Dr. Arthur Jey, an emergency physician at Sutter Health in Sacramento, told reporters that staying out of the heat is important. Wearing a hat and loose clothing, staying hydrated and watching for signs of heatstroke are also important.

“Heatstroke is like a stroke,” Jey said, describing symptoms as odd behavior, severe headache, blurred vision, heavy sweating and then no sweating.

“And that’s a really big deal,” Jey said. “So we want to prevent them from even getting close to heat stroke.”

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The heat in California was expected to spread from north to south through the week, with the worst concentrated inland, including the Sacramento and San Joaquin valleys and the southern deserts. But warnings extended just below the coast.

According to meteorologists, in San Francisco, known for its cool summers, the maximum temperature on Tuesday will be over 31 degrees Celsius in the city center, but even around 18.3 degrees Celsius in Ocean Beach.

“The high pressure dome will remain over California for at least another week. Longer-range forecasts suggest that planning may even be optimistic,” the Bay Area Weather Service wrote.

The heat was accompanied by gusty, dry winds in the state’s north, where utility Pacific Gas & Electric shut off power in 10 counties to prevent wildfires from downed or damaged power lines.

According to PG&E, about 12,000 customers were told their power might be shut off and were given information about centers where they could get ice, water, snacks, Wi-Fi and other supplies.

California has seen a series of wildfires in the spring and early summer that are feeding on abundant grasses created by successive wet winters. The largest current blaze, the Basin Fire, was 17 percent contained after charring more than 21 square miles (54 square kilometers) of the Sierra National Forest in eastern Fresno County since it ignited on June 26.

Antczak reported from Los Angeles. Associated Press journalist Christopher Weber contributed from Los Angeles.