The white arrow points to a southward moving outflow boundary laid down by earlier storms northwest of DC. As this image shows the boundary has already fired storms (cell just north of the center of the line) and the hope is that it will continue down this way and fire up cells around here. Anything to beat the heat!!!
7:07 pm UPDATE: Alas, the boundary died out as it reached the Stafford Courthouse area, along with the updrafts it was creating. Storms tomorrow?