PHOENIX >> In a desert city used to triple-digit temperatures, some hunker down inside and some head to the pool, while others still want to hit Phoenix’s many hiking trails. But with temperatures creeping above 115 degrees during a heat wave boiling parts of the Southwest, that decision can have deadly consequences.

The mercury made a quick ascent today to hit 118, breaking a record of 115 set nearly 50 years ago, according to the National Weather Service. Portions of Arizona and southeast California were expected to keep getting scorched Monday with a high pressure ridge lifting out of Mexico.

As of noon in the Los Angeles area, numerous areas had exceeded 100 degrees including Burbank, Beverly Hills, Woodland Hills, Fullerton, Glendale and Riverside.

The 106-degree reading in Burbank set a new record for the day. One National Weather Service weather site showed a 112-degree reading in Beverly Hills.

Going out early in the morning didn’t save a 28-year-old woman who became unresponsive while mountain biking with friends in north Phoenix. She had set out with two friends around 6 a.m., carrying water, but became exhausted about three hours later, 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 comes a day after a 25-year-old Phoenix man died of heat exposure while hiking in neighboring Pinal County.

Forecasters have said Phoenix could see a high of up to 120. On social media, residents commiserated 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 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 as the last day of spring brought summerlike temperatures to the region.

In Silver Lake, a fire at a home quickly spread to brush along the 2 Freeway, prompting firefighters to call for more resources and air support. The freeway was closed in both directions near its terminus in the Echo Park section.

Los Angeles Fire Department choppers were making water drops on a neighborhood along the freeway and firefighters were working in backyards with hoses trying to prevent structure losses.

The LAFD said about 150 firefighters were on scene, trying to prevent the fire from moving west of Silver Ridge Avenue and east of Corralitas Drive.

The National Weather Service issued a red flag fire warning for large parts of the region because of extreme heat, low humidity and the potential for gusty winds. The Sherpa fire in Santa Barbara County has already burned more than 7,800 acres, and firefighters have also been dealing with smaller blazes.

Forecasters said there was “potential for extreme fire behavior/rapid fire spread” of the Sherpa fire because of continued sundowner winds and hot temperatures.

Smoke from the fire is contributing to hazy conditions as far south as Los Angeles and Orange counties, according to the South Coast Air Quality Management District.

Temperatures are expected to rise Monday, the first day of summer, in the heat wave forecast to last through Tuesday.

The blistering heat is the result of an upper-level ridge of high pressure that is building slowly over Southern California. Joe Sirard, a meteorologist with the National Weather Service, said the system should start to weaken by Tuesday and temperatures should start to drop.

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The Associated Press and Tribune News Service contributed to this story.