Millions of blasting air conditioners strained electric grids, prompting Texas and California utilities to threaten shut-offs. The National Weather Service in Las Vegas tweeted all-caps appeals for residents to stay hydrated and stay inside: "Long duration heat waves are DEADLY." Doctors from Palm Springs to Phoenix warned about pavement so scorching it can give people third-degree burns.
The first major heat wave of the summer has seized the western United States, toppling records and threatening lives. The event is unprecedented in its timing, intensity and scope, said Washington State University climate scientist Deepti Singh; never have such severe conditions been recorded over such a large area so early in the summer.
At least four weather stations, including Salt Lake City, have matched their all-time high temperatures this week, months before the hottest part of the season. Las Vegas set a new daily record, at 116 degrees Fahrenheit. In Death Valley, Calif., thermometers read 126 degrees.
Coupled with a catastrophic drought that has damaged crops and shrunk vital reservoirs to all-time lows, the blazing weather is a trademark of human-caused warming, Singh said.
Unless the world drastically cuts greenhouse gas emissions, summer temperatures in the West could rise by as much as 6 degrees Fahrenheit by the middle of the century. The number of extremely hot days in the United States could triple, and the duration and extent of heat waves will become more extreme.
In the meantime, more than 40 million residents of Western states must endure a bout of broiling conditions that shows no sign of letting up. Temperatures are forecast to hit 110 degrees and higher this weekend in cities spanning the Southwest. Red flag warnings are in place across Nevada and Utah as dry lightning and gusty winds threaten to spark new wildfires.
People sought relief wherever they could find it: in the shade of trees, the spray of playground sprinklers, the air-conditioned lobbies of government buildings.
At Sacramento County's Department of Human Assistance on Thursday, 54-year-old Suzy Fobbins sat on a plastic chair beside everything she owned: a cart with clothes, a rolled-up tent, a sleeping bag and a cellphone.
Normally, Fobbins said, she lives in a parking lot in a different part of town. But she almost passed out inside her small tent when the temperature began inching up to 100 degrees earlier this week.
"I started retching," she recalled. "I couldn't see, I was so dizzy."
Fobbins has asthma and diabetes, and she knew she wouldn't be able to withstand Thursday's forecast of 109 degrees. So when a probation officer mentioned this refuge, she got on the bus.
She hoped she'd be able to get a motel room through a service operated by the Federal Emergency Management Agency.
"You can't live like that," Fobbins said. "A lot of people are going to die in this heat because they don't know where to go."
Sweltering nights are a trademark of climate change, which is increasing overnight lows at nearly twice the rate of daytime highs.
"It can be miserable," said Anna Bettis, who manages the Nature Conservancy's healthy cities program and lives in south Phoenix.
For the past two nights, temperatures in the city haven't dropped below 90 degrees. By morning, the sun feels like an assault.
"It's kind of like your skin is almost sizzling," Bettis said.
She worries about the toll of heat on the city's most vulnerable residents. Experts consider extreme heat to be the deadliest kind of weather disaster. Without water, shade and a chance to cool off, the body's ability to regulate temperature becomes overwhelmed - a condition known as heat stroke. Cells start to break and internal organs shut down. The threat is especially severe for people who are elderly, homeless or suffer from preexisting health problems, such as cardiovascular disease and diabetes.
Maricopa County in Arizona - one of few places to consistently track the health effects of heat - attributed more than 300 deaths to extreme temperatures last year. Since April, the county has recorded three heat-related deaths and is investigating 20 more.
A network of government agencies and community groups in the region has scrambled to turn shelters and offices into emergency cooling centers. At the nonprofit St. Vincent de Paul campus in south Phoenix, some 200 cots now fill what used to be a dining hall. Coolers by the entrance are stocked with water, Popsicles and ice.
According to the Centers for Disease Control and Prevention, farmworkers die of heat at roughly 20 times the national rate. But there is no national temperature standard for outdoor laborers; California is one of only a few states where farms are required to provide access to water and shade.
The Washington Post's Mark Kreidler in Sacramento, Ryan Slattery in Las Vegas and Jimmy Magahern in Phoenix contributed to this report.