The Southwest Braces for Record-Breaking Heat

People across the southwestern United States were trying to stay cool on Saturday as a punishing heat wave was posed to break temperature records.

The National Weather Service issued an excessive heat warning for this weekend that covered more than 31 million people, mostly in Arizona, Nevada and California. Another 61 million people on the West Coast and in the South were under a heat advisory.

The Weather Service forecast 45 record-high temperatures across the country ahead of the weekend. Most of the predicted record-breaking temps were in the Southwest.

Municipalities and community organizations scrambled to provide water and cool shelter to their most vulnerable populations, primarily homeless individuals.

In Las Vegas, temperatures were projected to reach 116 degrees Fahrenheit, and cooling centers and homeless shelters were seeing an increase in visitors.

Nearly all 500 of the beds in the Salvation Army’s overnight shelter there have been occupied in recent days as the sweltering heat has lingered into the night, according to Harold Laubach, a county coordinator with the organization in southern Nevada.

The Salvation Army has also offered a daytime cooling center in Las Vegas, which saw a daily average of 250 visitors last week. That figure has jumped to 300 visitors in recent days and continues to rise, Mr. Laubach said.

“There are cooling centers all across the southern United States right now,” Mr. Laubach said. “Many of them are doing the same thing, which is essentially keeping people alive and healthy.”

Heat is the leading weather-related killer in the United States, according to the Weather Service. Annually, roughly 67,000 people visit emergency rooms with heat-related illnesses, and about 700 people die of heat-related causes each year, according to the Centers for Disease Control and Prevention.

In Phoenix, where the temperature was forecast to reach 118 degrees Fahrenheit on Saturday, city officials reported a surge in heat-related 911 calls. On Thursday, the desert city matched a high with 33 such calls, David Hondula, the director of the city’s heat response office, said at a news conference on Friday. So far this year, 12 people have died heat-related deaths in Maricopa County, which includes Phoenix, officials said.

Some areas in the Southwest, such as Tucson, Ariz., also reported excessive heat warnings for longer stretches than usual.

“Typically we only have excessive heat for three, four, five days in a row,” Aaron Hardin, a forecaster for the National Weather Service in Tucson, said on Saturday. The current heat advisory, Mr. Hardin said, is set to expire Monday but could extend through midweek. He added that this month, an excessive heat warning had been in place for seven straight days, the longest ever issued by the station.