People cool off with a water mist along the Las Vegas Strip, Tuesday, July 24, 2018, in Las Vegas. The National Weather Service has issued an excessive heat warning for the Las Vegas valley. [Photo: AP/John Locher]

Humberto Berlanga, center, wipes sweat from his face while waiting in the heat in line at the Gold & Silver Pawn shop, Tuesday, July 24, 2018, in Las Vegas. The National Weather Service has issued an excessive heat warning for the Las Vegas valley. [Photo: AP/John Locher]

A hiker walks past a heat warning sign at dawn, Tuesday, July 24, 2018 in Phoenix. Much of Arizona and parts of California, Arizona and Utah are under an excessive heat watch during a week that forecasters say could prove to be the hottest of the year. [Photo: AP/ Matt York]