After studying video footage and radar images from Saturday's deadly stage collapse at the Indiana State Fair, AccuWeather.com Senior Meteorologist Henry Margusity suspects that a gustnado was a cause of the collapse.
"In the video, it appears that a possible gustnado traveled from left to right across the stage area," Margusity said. "The video shows a swirl of dust coming across the stage, and it's only when the swirl hits the stage that the stage actually collapses."
The National Weather Service defines a gustnado as a short-lived, ground-based, shallow vortex that develops along a gust front associated with either thunderstorms or showers. More information is provided at the bottom of this page.
Margusity also pointed out from the video more clues from a flag waving in the background.
"Typically, a flag that size needs winds of about 46 mph to wave around like that," Margusity explained. "Given that the flag pole was moving as well, winds were probably near 50 mph."
Wind gusts up to 50 mph were measured at Indianapolis International Airport, which is about 20 miles southwest of the fairgrounds. Radar-indicated wind speeds were 50 mph to the west of the fairgrounds about 20 minutes prior to the collapse.
Margusity said that changes in the orientation of the flag further supports the idea that it was a gustnado that came through.
"The flag actually turns toward the dust swirl after the stage collapsed and as the dust swirl moves past it," Margusity stated. "This indicates that a circulation of wind was moving through the area."
Taking a look at radar data from Saturday evening, a gust front can be seen propagating ahead of a line of thunderstorms approaching the fairgrounds from the west.
At 8:35 p.m. EDT, radar showed the thunderstorms collapsing and sending out a gust front 3 miles ahead of the storms.
By 8:44 p.m., the gust front was accelerating, traveling at a speed of 53 mph. At 8:48 p.m., it hit the stage.
The thunderstorms at the time of the collapse were still 10 miles away to the west and didn't hit the fairgrounds until 9:02 p.m.
Radar-estimated wind speeds could not be measured during the collapse because of radar clutter.
Other evidence supporting Margusity's gustnado theory are videos (below) taken by storm chaser Michael Moss in the Indianapolis area that showed several gustnadoes along a gust front.
Definition of a Gustnado, from the National Weather Service
A slang term for a short-lived, ground-based, shallow, vortex that develops on a gust front associated with either thunderstorms or showers. They may only extend to 30 to 300 feet above the ground with no apparent connection to the convective cloud above. They may be accompanied by rain, but usually are 'wispy', or only visible as a debris cloud or dust whirl at or near the ground. Wind speeds can reach 60 to 80 mph, resulting in significant damage, similar to that of a F0 or F1 tornado. However, gustnadoes are not considered to be a tornado, and some cases, it may be difficult to distinguish a gustnado from a tornado. Gustnadoes are not associated with storm-scale rotation (i.e. mesocyclones) that is involved with true tornadoes; they are more likely to be associated visually with a shelf cloud that is found on the forward side of a thunderstorm.
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From AccuWeather.com (find the original story here); reprinted with permission.