EDIT (7:45pm): At this time the Tornado Watch has been cancelled for the entire DC Metro Area. Isolated showers and storms are still possible thru this evening.
A Tornado Watch has been issued until 10:00pm for most of Maryland, DC, and Fairfax, Loudoun, and Arlington in Virginia.
This is a large threat area, and impacts could include numerous tornadoes, strong winds in excess of 70mph, and large hail.
Start making preparations now so you are prepared if severe weather reaches your area. Severe Thunderstorms have already been moving through parts of Western Maryland and West Virginia.
Keep an eye on the weather, and listen to NOAA Weather Radio and local media for updates, as conditions can and will change rapidly.