A powerful Mesoscale Convective System that originated in Des Moines IA, and followed I-80 well into Ohio. (note that I haven't heard this name repeated anywhere, it's the name I coined for it) Most notably it blew windows out of Chicago's tallest building, the Willis Tower and produced 90 mph winds over southwest Michigan.
Having seen it coming long before it made it to Chicago, I decided to set up a timelapse of the event. I'm recording for the extreme northern end of the Derecho where wind speeds were about 20% weaker than near the center of the Derecho. This is a good thing, down the road from me a weekend outdoor concert was taking place with more than 10,000 spectators with little or no cover available.
For a good portion of the video, you'll see the overshooting tops of the anvil approach and darken. As the storm finally gets closer, a shelf cloud appears with it's eerie rolling motion and quickly moves in. At this point I grab the camera and switch to realtime video to witness the 40-50 mph wind gusts that accompany the back side of the shelf cloud. Scroll down for more.
Unbelievably, another Derecho hit hours later over some of the same locations in Iowa, Illinois and Indiana. This time it missed most of Michigan, but nevertheless we had a great view of it. Nearly constant lightning was occurring over the Chicago area, but the frequency died down as it approached the South Bend, IN area. Here's another timelapse of the second event.