PHOENIX — The first day of summer brought some of the worst heat the Southwest U.S. has seen in years, forcing flights to be canceled, straining the power grid and making life miserable for workers toiling in temperatures that reached 120 degrees or higher in some desert cities.

Arizona, Nevada and California had dramatic temperatures Tuesday, and researchers say deadly heat waves like this one were going to grow more frequent.

The forecast called for a high of 120 degrees in Phoenix, which it hasn’t seen in more than two decades. Death Valley, California, reached 125 and Palm Springs hit 121, still a degree lower than the same day last year.

The operator of California’s power grid called on people to conserve electricity during peak hours.

Workers at a construction site in a Phoenix suburb huddled under an excavator to find a sliver of a shade during a break. At another building site, men in hard hats and yellow vests labored and sweated in the heat, downing water to stay hydrated. Project superintendent Tommy Russell says his company has held weekly safety meetings to prepare for the heat, and he will send his workers home if it hits 120.

Las Vegas also baked. Visitors tried to stay inside air-conditioned casinos, and some tourists lugged packs of bottled water around the Strip. Others went to a bar where the temperature is set at 23 degrees and glasses, walls and seats are sculpted from ice.

Tonya and Lavonda Williams traveled to Sin City from Orlando, Florida, to see the Backstreet Boys in concert. Walking on the Strip in 112 degrees was too much to handle, even for people accustomed to heat.

“This is like the oven door is open,” Lavonda Williams said as the sisters walked from a pedestrian bridge into The Palazzo casino-resort.

“It’s too hot to even drink alcohol,” Tonya Williams added.

Landscaper Juan Guadalupe scaled a spindly palm tree more than 50 feet tall in Phoenix, using a chain saw to hack the branches. He didn’t mind being tethered to a tall tree because he occasionally catches a cool breeze.

“Down here, it’s hot,” Guadalupe said.

With cooling and hydration stations in full swing across the region, hundreds flocked to Grace Lutheran Church in Phoenix for water and refuge.