An excessive heat warning — the National Weather Service's highest alert — is in effect for about 36 million people, or about 10% of the population, said NWS meteorologist Bryan Jackson. An excessive heat warning is issued within 12 hours of the onset of extremely dangerous heat conditions.

Temperature records have been shattered from California and Nevada to North Carolina in the last few days, and relief is still days away, forecasters say.

A widespread heat wave is expected to deliver a fresh batch of record temperatures along the U.S. West Coast, as millions of Americans sweat through a heat dome that is also hovering over Arizona and Nevada.