Dr. Matthew Brown Parker, associate professor of marine, earth and atmospheric sciences at N.C. State University, explains the meteorological event known as a gustnado (gust-NAY-doe) - which may have caused the collapse this month of a stage at the Indiana State Fair.
Q: What is a gustnado?
A gustnado is a small-scale vortex, a column of swirling air, perhaps 100 meters (328 feet) in width that occurs along the leading edge of a thunderstorm's gusty outflow. The term is not used all that often in public weather forecasts and warnings, probably because gustnadoes are generally short-lived, weak and very difficult to detect.
Q: What causes a gustnado, and how does it differ from a tornado?
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Gustnadoes form when shearing motion along a thunderstorm's outflow is combined with lifting motion along that thunderstorm's outflow. Both gustnadoes and tornadoes are associated with a parent thunderstorm. A tornado's source of spin is steadier, and its spin is substantially enhanced by upward motion throughout the entire depth of the parent thunderstorm. A gustnado's spin stretches upward through only a shallow depth.
Q: How common are they?
The ingredients needed to create a gustnado are much more common than the ingredients needed to create a tornado.
In theory, most thunderstorms are capable of producing something like a gustnado. But it's not known exactly how frequently gustnadoes occur; most of them are weak, short-lived, and go unobserved. Gustnadoes are only rarely accompanied by a funnel cloud; they are most likely to be noticed when they move over dry, bare ground (where some dust can be lofted and swirled). Meteorologists don't make specific predictions for gustnadoes, and the existence of a gustnado would not normally trigger a tornado warning.
Q: Do they usually cause damage, such as that seen in the Indiana stage collapse?
Gustnado damage tends to be marginal and transient (perhaps removing some tree leaves or downing a couple of branches). Top-end gustnadoes could approach the strength of a weak tornado, at least for a minute or two. In a densely populated area or a poorly-built structure, there is certainly the rare possibility for tragedies like what occurred in Indiana.
It's important to point out that many severe thunderstorms produce considerable wind damage due to causes other than gustnadoes. This is often called "straight-line wind damage," and its footprint would normally be much greater than the width of an individual gustnado.
Q: The North Carolina and South Carolina state fairs are in October. What is the seasonality of these high wind gusts (if any)?
The peak severe thunderstorm wind threat in North Carolina and South Carolina occurs in June and July, with at least a modest threat from mid-April through August. The threat in October is quite low. People need to stay abreast of severe weather forecasts and warnings.