August 13 Indianapolis State Fair Storm Stage Collapse Q & A

Above is a radar image at 7:57 CDT of Indianapolis. At the Indianapolis State Fair, hundreds  or even thousands were waiting for the band Sugarland to begin performing when the stage collapsed, killing 5 people and injuring dozens. So, what caused this tragic event to happen?

Q: What caused the stage to topple?
A: Look at the radar image above. Do you see a thin bright green line right in front of the storms? That is called 'outflow', or rain-cooled air. Like spilling liquid on a floor, thunderstorms will shoot down bursts of rain-cooled air that will spread out in all directions. If it is strong enough, it may even show up on radar, like it did last night. That outflow can be dangerous with very strong winds. As that outflow came through, the winds were so strong that the stage collapsed.

Q: How strong was the wind?
A: The National Weather Service estimated wind speeds at 60-70 MPH.

Q: Is outflow called a microburst?
A: A microburst is an especially strong blast of wind downwards from storms that can be considered life threatening. The term isn't widely used anymore, however. So specific types of outflow can be called a microburst, but rarely does that happen.

Q: Why didn't people just run away from the stage and go inside?
A: The outflow traveled so fast and came before the storm, so people watching the radar may have been unaware that the outflow had formed and was coming before the storm.