(CNN) More than 12 million US residents were living in areas under a tornado watch Thursday night, forecasters with the National Weather Service said.

Watches were in effect for parts of Maryland, North Carolina, Virginia, Washington D.C., and West Virginia.

The watch for Washington also included Maryland and Virginia. The weather service's Twitter account for tornado information said warnings had been issued in North Carolina and Virginia.

A tornado warning means one has been seen in person or on radar. A watch means conditions are favorable for a tornado to form.