Here's John's story of the day he took these photos:

"The storm I was chasing on this day was near Gregory South Dakota.  In order to intercept this storm I had to split between two supercell thunderstorms.  
 
I was leaving Sioux Falls that afternoon and driving west when the Storm Prediction Center issued a Tornado Watch for the area.  A while later, two supercell thunderstorms developed in Cherry County, Nebraska.  Both of these storms became tornadic.  The storm just southeast of Valentine, Nebraska was racing to the northeast.  At this time I was near Plankinton and I figured I could head south toward Gregory and possibly intercept it.  However, I knew I had to be careful because visibility would be rather limited approaching it from the North.  By the time we approached Platte, South Dakota, two supercells were going, one to my north and one to my south (the one headed for Gregory).  The one to the south was producing tennis-ball-size hail and the one to the north was producing quarter-size hail and eventually a tornado near Plankinton.  I approached the storm near Gregory from the north.  I stayed well out of the core of the storm, knowing that it was producing near baseball size hail.  I still managed to get pelted by quarter to half-dollar size hail and ended up with a few more dents in the truck.  Once the quarter-size hail started we drove north, trying to get out of it.  I called Chris Franks, who was working at KOTA that night, and he gave me an update on the storm track and the tornado-producing storm to my northeast.  We let things calm down a bit and drove back to the South to Gregory and that is where we came across these massive hail stones. 
 
I ended up getting back to Rapid City around 1 AM.  It was well worth the drive to see such an awesome storm."