For those going tonight (I won't be - DC United conflict) - from Yahoo! Weather:Today:
Some clouds and possibly an isolated thunderstorm this afternoon. A few storms may be severe. High 91F. Winds WSW at 5 to 10 mph. Chance of rain 30%. Tonight:
Variable clouds with thunderstorms, especially early. A few storms may be severe. Low 76F. Winds WSW at 5 to 10 mph. Chance of rain 40%.
Also, if you use Metrorail's Orange line, particularly from/to from West Falls Church, Dunn Loring or Vienna, budget more travel time tonight because of the Dulles Rail project (applies to tomorrow's finale too): http://www.wmata.com/about_metro/news/PressReleaseDetail.cfm?ReleaseID=4486