A dangerous heat wave, gusty winds and potential lightning strikes posed a critical fire risk for large parts of the Pacific Northwest on Sunday, as firefighters in Oregon battled wildfires that have burned over 400,000 acres in less than two weeks.

About 1.7 million people in Oregon and Washington State were under red flag warnings, the highest National Weather Service alert for conditions that may result in extreme fire behavior. It is issued when warm temperatures, very low humidity and strong winds combine to produce a heightened risk.

The Oregon State Fire Marshal’s office said on social media on Saturday that the next 24 to 36 hours would be “extremely challenging.” It asked residents to take precautions to prevent sparking man-made fires.

More than 11 million people in parts of Arizona, California, Idaho, Nevada, Oregon and Washington State were also under extreme heat warnings on Sunday morning. The authorities have warned that the heat could pose major health risks for residents who cannot rehydrate. The temperature in parts of the interior Pacific Northwest could reach over 110 degrees Fahrenheit on Sunday, the Weather Service said.

Several massive fires were already burning in central and eastern Oregon.

The Falls fire, which started on July 10 near the city of Burns, has scorched over 117,000 acres and was 12 percent contained as of Sunday morning, according to the Oregon Department of Emergency Management.

The Cow Valley fire, which started on July 11, had burned over 133,000 acres and was nearly 80 percent contained, according to the agency. The Lone Rock fire, which started on July 14, had burned over 100,000 acres and was 40 percent contained. And the Durkee fire, which started on Thursday, had burned over 74,000 acres and was zero percent contained.

About 2,800 people were under evacuation orders across Oregon as of Saturday, and another 2,700 were under evacuation warnings, according to the Department of Emergency Management.