We know what tornado and thunderstorm warnings are, even flash flood warnings. Do you know what a red flag warning is? Here's is the answer.

According to the NWS, a red flag warning is issued when the combination of dry fuels and weather conditions support extreme fire danger. For northern Illinois and northwest Indiana, the red flag warning criteria includes sustained 20-foot winds of 20 mph or higher, relative humidity less than 25% and 10-hour fuel moisture at 8% per day.

At least 9 million people in the nation's upper Midwest region are under red flag warnings on Monday, including residents of Minnesota, where a wildfire was already burning out of control in the largest national forest east of the Mississippi River.