Link to the Washington Post: Capital Weather Gang blog post. The progressive radar images montage is especially illuminating and frightening.
The continuous flickering lightning, with the lit-up sky looking almost rosy at times, was so freaky.
Actually, each of those parens is a snapshot of the same storm line over time. The video may make it more clear.
I'm bemused that I had never heard of a derecho before. I'm a moderate weather geek, ever since I was a kid, and working for 8 years at the American Geophysical Union (which has a big atmospheric sciences section) only exacerbated the situation.