Where Did This Come From?

My attention was squarely on the line of severe storms riding east along the Minnesota/Iowa state border around 6:30 pm CDT on Thursday, August 18, 2016. My intent was to capture some potential distant cloud structure by driving north on Highway 13 into northeast Iowa. Leaving Marion, Iowa, two small cells began to appear to my right (east), located along the Mississippi River just southeast of Bellevue, Iowa. As I continued my drive north to intercept the northern storms, the two twin cells grew. I thought, "Oh cool, that's a nice little scene..." The above image looks east at 6:49 pm on the big bend section of Highway 13 about 1 mile east of the town of Coggon in northern Linn County.

Stopping at one of my favorite spotting locations on 255th Street (D34) at Highway 13, about 4 miles south of Manchester in Delaware County, the two cells to the east had merged into one. The above image looks that direction at 7:15 pm, during the time the storm had become severe-warned. Note the strong "ice cream dollop" overshooting top at center, indicating its very strong updraft. The storm was now located about 68 miles distant, near the Elizabeth, Illinois area just east of the Mississippi River.

Briefly pulling up shop and driving east across Highway 13, I stopped here at a railroad crossing located on 255th Street about .7-mile east of the highway. Time was 7:21 pm. The overshooting top is partially obscured by a wisp of anvil cloud.

Back to my former spotting position. 7:27 pm. The mature storm's anvil has spread some 25 miles across. Though the line of (weak) storms--my original target--were at this moment advancing from the northwest, this storm cell stole the evening's show. What had been perceived by me as a superficial little pop-up cell to the east had become the main event! Nikon D7200 DSLR camera.

Radar screen capture of 6:50 pm (top panel) and again at 7:30 pm, showing the state of the eastern storm cell and my spotting position (target icon at left) in relation to it.