PHOENIX—People across the U.S. Southwest are cranking up their air conditioners on the first day of summer as they brace for more of the unrelenting heat that left two 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, and forecasters expect more of the same Monday, the National Weather Service said. Southeast California also faces triple-digit temperatures with a high pressure ridge lifting out of Mexico.

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

When it gets that hot, going outdoors early in the morning can still 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 she became exhausted about three hours later and then could not breathe.

Firefighters rescued the unidentified woman, who was an avid hiker and a personal trainer, and she later died at a hospital, fire Capt. Larry Subervi said. She had no known medical issues, and her condition appeared to be heat-related, authorities said.

Her death came a day after a 25-year-old Phoenix man died of heat exposure while hiking in neighbouring Pinal County.

The heat forced a Phoenix-bound flight to return to Houston on Sunday evening, United Airlines said. Passengers will be accommodated with an added flight Monday.

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

Some still ventured outside for quick errands. Kim Leeds, 28, had to take her dog Bo outside in the early afternoon for a bathroom break. The Australian shepherd wears special booties with rubber bottoms.

“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.

Southern Californians also posted competing photos on Facebook and Instagram of their soaring thermometers. Burbank and Glendale, just north of downtown Los Angeles, soared past 100 degrees by midday Sunday, the weather service said. Burbank saw a record 105 degrees.

Firefighters battling wildfires in the region were in for another day of brutal conditions, with Monday being the peak of the heat wave, forecasters say.

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Further northwest in the San Fernando Valley, some thermometers were reading close to 110 degrees, and Palm Springs in the inland desert hit 115.

The heat spurred state regulators to urge residents to voluntarily cut their consumption of electricity Monday. It did not bring the rolling blackouts that Southern Californians have been told to expect this summer after a massive natural gas leak.