The agency warned that extreme heat poses a danger in the evening as well, as temperatures are expected to remain above average overnight. As of Monday morning, nearly 8 million Americans were under heat alerts, according to the National Integrated Heat Health Information System.

Dangerously hot conditions hit states up and down the West Coast over the weekend, owing to a persistent heat wave stretching from the Pacific Northwest to California and the desert Southwest.

Parts of California, Oregon and Washington state will experience extreme heat at least through Tuesday, forecasters say.