SAN FRANCISCO — Thousands of people in wine country were without power Thursday amid a fall heat wave that brought another round of extreme wildfire danger to large portions of already battered Northern California.

Pacific Gas and Electric cut power starting Wednesday evening to more than 45,000 customers — about 100,000 people — mainly in the Sierra Nevada foothills and the San Francisco Bay Area. Some restorations began Thursday afternoon, and by evening the number of customers without power was about 30,000, the utility said. All power was expected to be restored by Friday night.

Many in wine country north of San Francisco were feeling drained by what seems like a never-ending wildfire season in the region.

Kathleen Collins has had to evacuate her home in the mountains of Napa County four times in the past five years because of fires. This summer, she lived in a motel for two weeks after leaving her home when a massive cluster of fires reached her tiny community of Pope Valley.

“It’s all very stressful. People are not happy, but there’s not much they can do about it,” said Collins, assistant manager at Silverado Ace Hardware store in Calistoga, a Napa County town of 5,000 people who were allowed to return home just last week after the Glass fire forced them out last month. The blaze that ravaged areas of Napa and Sonoma counties was contained Wednesday after destroying more than 1,500 homes and other buildings.

People have been buying generators, electrical cords, flashlights, batteries, gas cans and other supplies to help them deal with the latest outage, expected to last through Friday evening, Collins said.

More than 8,500 wildfires have burned more than 6,400 square miles (16,000 square kilometers) in California since the start of the year. Most have occurred since mid-August. Thirty-one people have died, and more than 9,200 buildings have been destroyed.

In Southern California, a brush fire Thursday near Redlands triggered a small evacuation as it grew to more than 100 acres (40 hectares). It was about 50% contained.

Collins said she received a message alerting her that her home would lose power Wednesday night and she dreaded having to throw away all her refrigerated food as in past outages. Fortunately, she hadn’t lost electricity as of Thursday.

which state officials ordered after last year’s missteps.

“These are challenging times. Not only are we right in the peak of the wildfire season … many of us are working from home, schooling from home as well,” he said. “We recognize that hardships are introduced when we shut off power.”

Barbara Nemko, Napa County’s superintendent of schools, said the warnings of high winds and intense heat sent “a shiver down my spine.”

“After undergoing two major fires in the last two months it’s almost unthinkable that we are having to prepare yet again for the possibility,” Nemko said.

She said the district has been fortunate that no public schools burned this year. Still, the costs have piled up as schools had to throw out large quantities of spoiled food, buy pricey filters to address terrible air quality and ash in the air, and secure generators to deal with outages.

Numerous studies have linked bigger wildfires in America to climate change from the burning of coal, oil and gas. Scientists have said climate change has made California much drier, meaning trees and other plants are more flammable.

Hot winds up to 55 mph were expected to heighten the threat of blazes throughout the San Francisco Bay Area, which has already seen some of the worst wildfires in state history. The National Weather Service issued heat advisories through Friday, with temperatures expected to reach triple digits in many parts of the state.