PHOENIX (AP) – People across the Southwest were cranking up their air conditioners Monday – the first day of summer – as they braced for more of the searing heat that left four people dead in Arizona over the weekend.

The mercury hit 118 in Phoenix on Sunday, breaking a record of 115 set for the date nearly 50 years ago.

In Southern California, the National Weather Service reported 17 daily heat records were broken on Sunday, most for readings well over 100 degrees.

The aptly named desert town of Thermal about 25 miles southeast of Palm Springs saw a high of 119 degrees. Burbank was a record 109, breaking the old mark of 104.

More of the same was in the works for Monday due to high pressure over the Four Corners region of the Southwest.

“We’re expecting something similar with highs around 115 to 120, with the warmest areas in Southern California,” National Weather Service meteorologist Bianca Hernandez said.

In Arizona, two hikers were found dead Sunday on separate trails near Tucson.

A 19-year-old woman died from apparent heat-related illness after she and a companion got lost, authorities said. Another man was found four miles from a trailhead. Rescuers were searching Monday for someone from the same hiking group.

In such extreme heat, even going outdoors early in the morning can turn deadly. A 28-year-old woman and two friends set out mountain biking around 6 a.m. Sunday in north Phoenix and carried water, but the woman was exhausted about three hours later and could not breathe.

The avid hiker and personal trainer was taken to a hospital, where she later died, fire Capt. Larry Subervi said. She had no known medical issues, and her condition appeared to be heat-related, authorities said.

A day earlier, a 25-year-old Phoenix man died of heat exposure while hiking in Pinal County.

On social media, residents commiserated about the sweltering heat by posting photos of boiling temperature readings on car thermometers and cellphones.

Some people ventured outside for quick errands. Kim Leeds, 28, had to take her dog Bo outside in the afternoon for a bathroom break. The Australian shepherd wears special booties with rubber bottoms that help protect his paws from hot pavement.

“He does really well with them. He doesn’t mind walking around,” Leeds said.

Preparing to enter her fourth summer in Phoenix, Leeds also decided to experiment with her car.

“I’m totally reveling in this experience because I’m actually baking cookies in my car,” Leeds said. “I’ve been here long enough that I’ve got to do these things.”

Others took advantage of discounts at Phoenix-area resorts, where summer can be the slow season.

Hotel guests got an early start Sunday at the JW Marriott Scottsdale Camelback Inn Resort and Spa as temperatures climbed. Several guests swam in the pool and sat under umbrellas sipping water and other iced drinks to cool off.

The National Weather Service said the heat broke a number of records Sunday in Southern California, where firefighters battling wildfires in the region endured another day of brutal conditions.

State regulators urged residents to cut consumption of electricity Monday, when the heat wave was expected to peak.

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Associated Press writers Andrew Dalton in Los Angeles and Anna Johnson contributed to this report.