Officials are blaming surprise wind gusts for the stage collapse that killed at least five people Saturday evening at the Indiana State Fair, basically saying the accident was unavoidable. I'm going to hazard a guess that a few civil lawsuits will claim otherwise very shortly. But regardless of whether nature or humans are at fault here, what actually blew the stage down?
The likely culprit is an "outflow boundary," a vast movement of energy created when a thunderstorm's updrafts and precipitation cool air so much it drops rapidly from the sky, hitting the ground and rippling outward. These cold-frontlike boundaries can move hundreds of miles away from the storms that generated them; in the case of Indianapolis, a severe-weather system was hovering very near the Indiana State Fair when wind, rain and dirt started blowing as fast as 70 m.p.h. to topple the stage.
You can follow police and fire activity in the wake of the horrible accident in the below video, which features chatter from emergency responders spliced over footage of the wreckage.