Is a gustnado dangerous?
A gustnado spun up in Louisville, Kentucky
For some reason we love to combine words, even if they have the same meaning. For example giant + enormous = ginormous. The weather world has one of these terms, gust + tornado = gustnado. But unlike “ginormous” the word “gustnado” is more gust and less tornado.
This Memorial Day a gustnado formed near the intersection of Hwy Q and Hwy 164 on the Waukesha/Washington county line. It caused minor damage, took down part of a fence, above ground pool, flipped a portable basketball hoop, etc. The gustnado had a few witnesses who reported what they saw as a possible tornado. It’s understandable since near the ground it looks like a tornado, spins, like a tornado, and does damage like a (small) tornado. But this gustnado, like most, was the twisted part of a gust front or straight-line winds flowing out from a nearby thunderstorm.
(Below soffit damage from the gustnado 5/28/2012. Photo by Tom Edwards)
The best description of a gustnado is a dust devil on steroids. The whirling wind is due to friction and low level wind shear, not due to a fast rotating mesocyclone at the heart of a massive thunderstorm. Think about the cool rush of wind that pours out the bottom of a thunderstorm. The winds fan out but don’t always move at the same speed. Part of the gust front could be moving at 50 mph while another portion may be slowed to 35 mph as the wind is slowed by friction, trees, buildings, or changes in elevation. The difference in wind speeds leads to wind shear causing an eddy at the leading edge of the fastest winds. The swirling eddy is what turns into a gustnado spinning along the ground with the gust front. Still having trouble getting the picture? Stick your open hand in a swimming pool or sink full of water. Move it like a canoe oar. You’ll notice little whirlpools forming on the surface of the water. The whirlpools spin fast, faster than your hand skimming the water. In this case your hand acts as the driving force behind the mini “gust front” you’ve created in the water. And along the edge of the fastest moving water you created mini gustnadoes or in this case whirlpools.
Gustnadoes typically stay close to the ground anywhere from 50 feet to 300 feet tall. Since they aren’t connected to the parent thunderstorms the damage they do is recorded as thunderstorm wind damage and not tornado damage. Gustnadoes can spin as fast as an EF-0 or EF-1 tornado which can do enough destruction for an insurance claim. In rare cases injuries or death occur. On August 13, 2011 a music stage at the Indiana State Fair collapsed as a gust front from storms 10 miles away hit the area. The collapse killed 7 and injured 43. Some meteorologists believe a gustnado actually hit the stage. Youtube clips of the event show a plume of dust arriving at the stage as it falls, but whether or not the plume is swirling in a gustnado is still up for debate (Some also say the American flag in the background changes direction which would be a clear sign of rotating winds as opposed to straight-line winds).
Gustnadoes don’t live long, don’t move far, and are too short and narrow to be picked up by radar so detecting them before they happen is very difficult. However, picking up the development and movement of a strong gust front is much easier giving us a clue as to where the next gustnado may occur.