The Indiana Fair Weather Tragedy

I was just reviewing the weather radar images from the recent tragedy at the Indiana State Fair where a sudden strong gust of wind blew stage rigging down onto fans waiting for a Sugarland concert. 

From the radar images it was pretty obvious that a “gust front” or “outflow boundary” from severe thunderstorms struck the fairgrounds before the main wind and rain arrived from the thunderstorms themselves.

So--what is a gust front or outflow boundary and how does it form? 

All of us are familiar with cold fronts that when they pass through Washington can bring wind, rain and cooler temperatures.  These fronts are on a giant scale of “many states long” and are caused by colder air moving down from the arctic in the winter.

Gust fronts or outflow boundaries are small-scale cold fronts (less than a state long) that are not fueled by cold air from the north, but by local thunderstorms. 

If you have been following my last two blogs you learned that cold air can sink in the atmosphere but warms by compression and most of the time becomes too warm from compression and stops sinking.  Occasionally, air that is cooled enough can plunge to the ground and rapidly spread out as a small-scale cold front with high winds, but not rain.

How does the air get cooled enough to plunge to the surface?

This is the time of the year for outdoor swimming, but coming out of the water (especially when the wind is blowing) can give us a chill.  The water evaporating from our skin cools us down. The same effect happens when raindrops fall into dry air and evaporate.  The evaporating raindrops can cool the air.  Often, just before it rains here, cool gusts can be felt from that effect.

In a severe thunderstorm, gobs of raindrops can be thrown out into very dry air located far above the ground where their evaporation can rapidly cool the air.  If this cooling is enough, the cold air (even though it is warming by compression as it falls) always stays colder than the air around it and can reach the ground spreading into a gust front.

The winds along a gust front can be intensified under the right conditions (what the winds are like the gust front moves into) by the formation of “gustnadoes”.  These are small tornadoes that sometimes form along the gust front and can increase wind damage in very local areas.

Could this tragedy have been avoided by better forecasting? 

The gust front on the radar images would have been apparent to a trained observer, so the forecasters at the weather service would have seen it.  Other people not trained would have not noticed it, and most likely would have been concentrating on the main weather from the severe thunderstorms that arrived later then the gust front. 

Gust fronts are very shallow and often the strongest winds are below the reach of weather radar untilit is too late (yes--the earth is round and radar beams are straight).  Also, it is almost impossible to forecast where “gustnadoes” might form under such circumstances.