They don't really know. There are thunderstorms all over the midwest every week this time of year. They come in lines, stretching for hundreds of miles from SW to NE, perhaps thirty miles deep. Most of them are completely innocuous - even enjoyable - but tornadoes can occasionally develop (and disappear again) in an instant. These guys got the warning that one had developed just minutes before the it struck. Being Boy Scouts, they probably did all they could to move toward safe shelter.
This is an especially bad spring - my town is not threatened, but nearby many rivers are in greater flood than has ever been recorded. In Iowa City, where the crest is expect on Sunday, it's anticipated ...

They don't really know. There are thunderstorms all over the midwest every week this time of year. They come in lines, stretching for hundreds of miles from SW to NE, perhaps thirty miles deep. Most of them are completely innocuous - even enjoyable - but tornadoes can occasionally develop (and disappear again) in an instant. These guys got the warning that one had developed just minutes before the it struck. Being Boy Scouts, they probably did all they could to move toward safe shelter.
This is an especially bad spring - my town is not threatened, but nearby many rivers are in greater flood than has ever been recorded. In Iowa City, where the crest is expect on Sunday, it's anticipated that the river will be five to ten feet deeper than the previous all-time record. Last night in Cedar Rapids, staff began evacuating a hospital that had only one dry entrance. The business district (it's a city of 100k) is accessible only by boat. Many from here have taken the day off to drive to nearby towns and fill sandbags for artificial levees. I've never seen anything like it.