On the evening of June 11, 2008, more than 100 Boy Scouts, teenage staff, and adult leaders were scattered around the wooded grounds of the Little Sioux Scout Ranch near Little Sioux, Iowa. Halfway through a weeklong leadership training program, the scouts had just finished eating a spaghetti dinner, and many were lingering near the camp's north and south cabins, designated as storm shelters, due to inclement weather. Nathan Dean, the camp's live-in ranger, was using the Internet to carefully monitor the weather from his house near the ranch's entrance and was corresponding with Dr. Dennis Crabb, a scoutmaster stationed inside the camp's headquarters one-half mile away.
Around 6:15 p.m., the pair received word via the camp's NOAA weather radio that a severe thunderstorm warning had been issued for their area. A few minutes later, Dean received a call from his father-in-law saying there was reported rotation near the town of Little Sioux, 5 miles to the southwest. Dean hurried outside to assess the situation, and, although he could only see what appeared to be a large, gray wall of rain, immediately radioed Crabb and told him to get the kids inside the cabins. Crabb sounded the camp's single siren, an aluminum 1950s civil defense–era relic that he had purchased, refurbished, and given the ranch as a gift only 2 months earlier.