On the afternoon of May 25, 2008 -- the Sunday of Memorial Day weekend -- a dangerous mix of meteorological forces began to coalesce just outside Parkersburg, Iowa. The storm came together at an alarmingly fast rate. Chief Deputy Tim Wolthoff of neighboring Grundy County was on duty that day, patrolling the area for storms, when this photo was taken. "As we got further north, then you could see that it was starting to get pretty dark," he said.
Watch the storm take shape on "20/20" FRIDAY at 10 p.m. ET
(Courtesy Tim Wolthoff)