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.

Red flag warnings now cover parts of Oregon, Washington and California. In Oregon and California, crews are battling flames that are burning thousands of acres. Along the West Coast, heat alerts remain in effect for 11 million people.

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