Excessive heat warnings have been issued across much of the U.S. Southwest, as well as southern California, where dangerously high temperatures have already led to the deaths of at least four people.
New wildfires erupted Monday west of Santa Barbara, California, while other blazes burned across Arizona and New Mexico.
California's power grid operators warned people to conserve electricity to counter the rising demand for air conditioning, urging consumers to wait until after dark to run major appliances.
Four people died from the heat Sunday in Arizona. All were outside, either hiking or mountain biking, when they suffered from heat exposure. At least three of those who died were in their teens or twenties.
Many cities in California reported record highs Sunday and Monday, with temperatures rising well above 38 degrees (100 F). The desert city of Palm Springs saw a high of 48 degrees (118 F).
Forecasters expect Monday to be the peak of the heat wave, with temperatures dropping somewhat by Tuesday.