There was some uncertainty about just how hot it was at 4 p.m. local time -- the National Weather Service's thermometer topped out at 128 degrees, tying the mark for the hottest June day anywhere in the United States. But the Los Angeles Times reported a National Park Service thermometer 200 yards away recorded a temperature of 129.9 degrees.
The newspaper said the weather service has the final word on such records and its official electronic readings will not be available until 8 a.m. Monday.
"There's only one thermometer that counts," Charlie Callagan, the park's wilderness coordinator and former head of its weather station, told the Times. "There's a possibility that we actually broke the record for June, and that's exciting."
The heat wave across much of the U.S. Southwest could lead to a fireworks ban just days before July 4 celebrations, officials said.
NBC News reported Las Vegas tied its all-time high of 117 degrees Sunday, equaling the temperature reached July 24, 1942 and July 19, 2005.
The heat and accompanying wind helped spread a wildfire near Yarnell, Ariz., where 50 homes were evacuated Sunday ahead of the flames, NBC said.
The network said triple-digit temperatures caused at least a dozen marathon runners in Pasadena, Calif., to experience heat-related illnesses.
Lancaster, Calif., set a record of 111 degrees Saturday, CNN reported.
Residents throughout Arizona, Nevada, Utah, Idaho, Oregon and California were encouraged to pay heed to extreme heat warnings and stay inside in air-conditioned locations until the temperatures moderate sometime this week, CNN said.
"People driving through desert areas during the pattern should make sure their vehicle can make the journey and that they carry extra water in case their vehicle breaks down," Western weather expert Ken Clark said on Weather.com
The heat was caused by a high-pressure dome blocking cooler air from coming down from the Pacific northwest, CNN reported.
The dry, hot conditions could spark thunderstorms that are unlikely to produce rain, but could include lightening that could spark wildfires, Weather.com reported.
The unusually high temperatures are expected to last through Wednesday, at which point they will begin dropping toward more normal levels, said Chris Stachelski a meteorologist with the National Weather Service in Las Vegas.
US Airways canceled 18 flights Saturday due to the high temperatures, spokesman Todd Lehmacher said. Airplanes are only allowed to take off in temperatures up to 118 and it crept up to 119 degrees in Phoenix, CNN said.