A heat wave building in Southern California is expected to peak Thursday through Saturday, breaking a run of unseasonably mild summer weather and creating a prime environment for wildfires to spread rapidly.

“This is the first major heat wave of the season, and it’s the biggest fire risk we’ve had,” said Kristen Stewart, a fire weather meteorologist at the U.S. Forest Service.

A flurry of red flag warnings were already in place, and state officials have sent a surge of firefighting crews and equipment into Southern California, where the memory of deadly fires that erupted in January still hovers in the background of everyday life. State and local officials there attempted a delicate balance of reassuring skittish residents without raising questions about the level of preparation for ferocious blazes in the past.

Gov. Gavin Newsom announced on Tuesday that he had directed Cal Fire, the state’s firefighting agency, to “pre-position” 10 fire engines, two hand crews, three dispatchers and a helicopter, among more people and equipment, in Los Angeles County.

By the end of the day, he said he was sending even more resources to five counties in Southern California — Los Angeles, Kern, Ventura, Santa Barbara and San Bernardino — in response to the fire risk, bringing the total to 32 fire engines, nine hand crews, 13 dispatchers and five helicopters, as well as other personnel and equipment.

“Now is the time to prepare, California,” he said in a statement.

California has experienced an average summer fire season, with only one blaze, the Gifford fire, surpassing 100,000 acres. But the hot weather combined with dry vegetation will heighten the risk of wildfires this week. And a chance for scattered thunderstorms across desert areas and the mountains could bring a mixed bag: Lightning could spark fires, but rainfall could lessen the danger overall.

“We haven’t seen a lot of fire activity this summer, but that could change this month,” Ms. Stewart said.

How hot will it get?

Temperatures in Southern California are likely to be 10 to 15 degrees above normal. Extreme heat warnings, the National Weather Service’s highest level of heat alert, were issued across the region, including in Downtown Los Angeles, where temperatures could persist in the 90s for four, maybe five, consecutive days.

While the heat will be significant and potentially dangerous, it’s not expected to break records.

Lisa Phillips, a meteorologist with the Weather Service office in Los Angeles, said heat-related effects may be intensified because most regions of the state haven’t experienced a major heat event this year and people may have not acclimated. Also, temperatures could stay warm overnight, with lows in the 70s and 80s in the hottest areas, not giving people a chance to recover.

Amid a changing climate, heat is becoming increasingly extreme and deadly. Last year, a study found a 117 percent increase in heat-related deaths from 1999 to 2023, with a particularly notable increase since 2016.

State emergency officials emphasized that their response to the potential for fire was not unusual, given the weather conditions expected. Mr. Newsom has said repeatedly that Cal Fire is one of the biggest and best-equipped firefighting forces in the world. And officials said that they were constantly evaluating where there is the greatest risk that fires could spread out of control at any given moment and were sending firefighters and equipment to those locations ahead of time to be able to respond more quickly.

“This proactive approach has proven to be a critical component of California’s wildfire response strategy,” Ed Chapuis, a spokesman for the governor’s office of emergency services, said. “It’s important to note that Cal O.E.S. has consistently used pre-positioning as a proactive firefighting technique for more than five years and this week’s mobilizations are part of an established deployment package.”

In Los Angeles, Mayor Karen Bass spoke Tuesday at a news conference alongside local fire officials, where they outlined the steps they were taking to quickly respond to fires, including clearing brush to ensure fire roads are clear.

Ms. Bass said no red flag warnings were in effect within the city of Los Angeles.

“However, I want to assure Angelenos that we are prepared,” she said.

The mayor also urged residents to prepare for the heat, which is in some ways even more dangerous than the threat of fire. The city has opened cooling centers and would be reaching out to the thousands of Angelenos who live outdoors.