You can read more about Chris's chasing adventures on his website: http://sky-chaser.com/
Here is a storm relative velocity image from the Aberdeen, SD (KABR) radar after the storm had moved just East of Bowdle. Note the strong rotation near the bottom center of the purple-colored tornado warning polygon. (Remember that green and blue colors depict motion toward the radar, while red and orange colors depict motion away from the radar; the radar is located at the right center portion of the screen).
Here is a reflectivity image of the same storm, taken about an hour earlier. Note the classic "hook echo" configuration:
Here is what the Aberdeen storm report log looks like (note that the log was sorted by tornado reports first, so it is not necessarily in chronological order):