This week, temperatures will once again scorch the western US, with an extreme heat wave that could bring the hottest temperatures of the summer so far.

Extreme heat warnings were in effect across parts of Southern California, Arizona and Nevada, affecting tens of millions of people. The extreme weather was expected to peak on Wednesday and last into the weekend.

The city of Los Angeles could see temperatures approaching 100F (37.7C), with locations farther inland reaching near 110F (43.3C) or higher, according to a forecast from the National Weather Service (NWS). “Dangerously hot conditions with highs of 95 to 110 possible, warmest Thursday and Friday. Warm overnight lows will provide little relief from the heat,” the NWS warned in an alert.

Desert cities like Palm Springs are expected to experience several days of temperatures above 43 degrees Celsius, while highs in Death Valley National Park, which is currently experiencing its hottest summer on record, are expected to reach 47.7 degrees Celsius on Friday.

Unusually high temperatures were also forecast for the Bay Area and Central Valley. Even normally temperate San Francisco was expected to be at least 55 degrees above average. It was so unusual for the city that the National Weather Service issued a heat warning.

“We’re talking about a solid four days of heat,” meteorologist Mike Wofford told the Los Angeles Times. “We’ve had three or four days of hot weather before, but this is hotter and longer lasting than most other heat waves we’ve had.”

The heat wave will add to the pain of an already devastating summer in the region.

Californians just experienced their hottest July on record, with an average temperature of 81.7F (27.6C) across the state. Many cities endured multiple days of temperatures over 100F (about 38C), and several cities broke temperature records during a notable July heat wave.

There’s little relief for residents in other states, particularly the U.S. Southwest. Las Vegas, Nevada, also experienced its warmest July on record, breaking a daily temperature record when the city reached 120F (48.8C). Meanwhile, Phoenix, Arizona, celebrated its 100th consecutive day with temperatures above 100F on Monday, breaking a streak that stretched into the 90s.

The scorching heat left the West primed for an explosive fire season after a wet winter blanketed the landscape with grass and vegetation that quickly dried into ready-to-use tinder.

Fire season is still in full swing, and September’s heat wave could make matters worse. Oregon has seen more fires this year than ever before, with nearly 1.5 million acres (607,028 hectares) scorched by mid-August. Dozens of wildfires continue to burn from Washington to Idaho to Arizona, while California has been battling the fourth-largest wildfire in history, the Park Fire, since mid-July, though that blaze is now nearly contained.

There has been some relief from the extremely high temperatures in recent weeks, but firefighters are still concerned that major forest fires will continue to rage in the west this fall.

“We’re about halfway through our season and it’s very busy and (we’re) getting the crew into that mental and physical mindset that we’ve got three months to go,” Dan Mallia, a firefighter who works with a special “hotshot” crew for the U.S. Forest Service, told the Guardian last month. “We’ve just been doing back-to-back assignments,” he said. “It’s been challenging.”