An outflow boundary or gust front in advance of an approaching squall line of severe thunderstorms swept through Indianapolis Saturday evening, toppling a stage onto concert goers, leaving five people dead and at least 45 others injured.
According to the National Weather Service (NWS), the gust front contained winds of 60 to 70 mph and aided in the collapse of stage rigging at the Indiana State Fair.
Officials on the scene of the tragic collapse reported that they were monitoring the weather conditions through an application on their smartphones as the storms approached but what they failed to see was the outflow boundary that they would have only detected if they had any type of meteorology training.
An outflow boundary or gust front is a small scale boundary separating thunderstorm-cooled air (outflow) from the surrounding air. Thunderstorms create this rain-cooled air as a result of evaporational cooling. Because cold air is more dense than its surrounding airmass, the rain-cooled air violently rushes toward the ground and spreads outward, creating downburst of strong winds.
These outflow boundaries can be seen either as fine lines on weather radar imagery or else as arcs of low clouds on weather satellite imagery. From the ground, outflow boundaries can be co-located with the appearance of roll clouds and shelf clouds.
The boundaries of wind can be out in front of thunderstorms by several miles. This could have made people think they had more preparation time than they did, because it preceded the rain and lightning by 10 or 15 minutes.
The Storm Prediction Center had issued a Severe Thunderstorm Watch for Central Indiana including the Indianapolis area from 6 pm EDT Saturday evening to 1 am EDT Sunday morning for the potential of damaging winds with storms.
The severe line of storms moved into the Indianapolis area after 830 pm EDT, prompting the NWS to issue a Severe Thunderstorm Warning from 839 pm to 945 pm EDT.
The strong winds estimated up to 60 and 70 mph occurred from 850 to 855 pm EDT, when the deadly collapse occurred.
Dramatic video of the deadly collapse into the crowd was captured.
Strong outflow winds such as what was experienced in the Indianapolis area is very common with summer thunderstorms.
A similar situation occurred across Mississippi in mid-June when strong winds out ahead of a complex of thunderstorms surged across the state from northwest to southeast, causing widespread damage and leading to the deaths of two people.
Just yesterday, strong downburst winds in excess of 50 and 60 mph with a severe storm downed multiple trees and power lines in the Vicksburg area, leaving one person injured.