The high pressure system bringing the relentless heat prompted the weather service to issue an Excessive Heat Warning beginning Monday at 10 a.m. and remaining in effect through Thursday at 8 p.m.in the Coachella Valley, San Gorgonio Pass near Banning and Riverside County Mountains and valleys.
High temperatures are expected to range anywhere from 96 to 98 degrees in the inland region Monday.
The National Weather Service said Waco reached 114 degrees around 5 p.m., beating the city's previous all-time record of 112 degrees set August 11, 1969.
Early Tuesday afternoon, temperatures had reached 118 degrees in Thermal and Palm Desert, and 117 in Palm Springs and Thousand Palms.
Thursday is set to be a "transition day", as temperatures begin their slow decline to more typical highs, he said.
Parts of Utah were also issued an excessive heat warning with temperatures this week expected to approach 109 degrees (43 Celsius). Officials expect Austin's Camp Mabry weather station to record reach record highs Monday.
The weather agency uses a complicated formula that varies from region to region to declare an excessive heat warning, including close to record-breaking temperatures.
Forecasters say monsoon moisture will slowly return to the Phoenix area from the east by the weekend.
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California's power grid operators are calling for voluntary electricity conservation as parts of the U.S. Southwest brace for another day of scorching heat.
The advisory will apply to almost all portions of southwest and south-central Arizona and will last until 8 p.m. Wednesday.
The Los Angeles County Department of Public Health also urged people to stay out of the sun.
In addition, young children and pets should never be left unattended in vehicles during hot weather, when auto interiors can reach lethal temperatures in a matter of minutes, officials warn.
Some of the highest temperatures over the next few days were expected in Phoenix, where the weather service issued a tweet to warn residents to stay hydrated and take advantage of cooler indoor buildings.
The thermometer hit 116 degrees a little after 3 p.m., breaking the record for the date of 115 set back in 1943.
Maricopa County public health officials say 155 people died in the Phoenix area a year ago from heat-caused illnesses such heat stroke.
"The numbers are definitely under what they were at the same time past year, so that's a positive sign", Fowler said.