it's just scattered storms just like it is every day all summer in D.C. when it's this hot and unstable.
Pretty much what Yahoo! Weather has tabbed for tonight:
'Scattered thunderstorms. A few storms may be severe. Low around 75F. Winds SSW at 5 to 10 mph. Chance of rain 60%.'
...in other words, more or less what they've predicted the past few nights.