The Black Cove Complex burning Thursday in western North Carolina is now the “highest priority fire in the U.S,” the state Forest Service said as millions of residents across the parched region remained under alert for wildfires.

Hundreds of Polk County residents have been forced from their homes for several days as the fire has spread into neighboring Henderson County. The National Weather Service issued a Red Flag Warning for the mountains of Polk and Henderson counties, indicating the potential for severe fire behavior and the possibility of rapid fire spread. 

Firefighters continue preparing firebreaks and protecting homes and businesses in anticipation of elevated temperatures, high winds and low humidity, the Forest Service said.

Elsewhere, two fires in South Carolina grew rapidly Wednesday, and parts Georgia, West Virginia and Tennessee were also are threatened.

“ALERT: There is an elevated fire danger across Southeast South Carolina and Southeast Georgia away from the immediate coast again this afternoon,” the Weather Service in Charleston, South Carolina, warned in a social media post Thursday. “A Burn Ban remains in effect for all of South Carolina.”

What is a red flag warning?

The National Weather Service issues Red Flag Warnings and Fire Weather Watches to alert land management agencies about the possibility of weather and fuel moisture conditions that could lead to rapid or dramatic increases in wildfire activity. This could be due to a combination of low relative humidity, strong winds, dry fuels, or any combination.