Both derechos and tornadoes are weather terms derived from Spanish given to storms that create damaging winds. The destruction left behind from a derecho can often look like that of a tornado, but the two are very different animals. Whereas tornado is Spanish for “to turn”, derecho is Spanish for “straight”, both referencing their winds.
A derecho is a long lived, typically bow-shaped complex of thunderstorms that produce damaging straight-line winds. Derechos can produce weak tornadoes, but they are mainly known for producing strong straight-line winds due to their structure.
Most times, a derecho starts out just as a cluster of thunderstorms in a relatively low directional shear environment. Tornadoes like the turning of the winds with height in the atmosphere; derechos like the winds to be in the same direction. This helps to push it along.
As thunderstorms in this cluster start to rain, the cooler air that comes down from the cloud with the rain pushes out into the usually warmer air, creating newer storms. These newer storms again have the rain cooled air that pushes out, known commonly as the gust front, and as you can imagine, the process reinforces itself. This is why derechos can travel for so long, they essentially become self-sustaining. This constant building up of newer storms along the edges give them their usual bow shape, and can lead to even faster movement and stronger winds.
Like a blizzard, a wind storm has to meet certain criteria to be considered a derecho. Those criteria are:
- Wind damage swath extends more than 240 miles (400 km)
- Wind gusts of 58 mph or greater along most of its length
The criteria to reach derecho-level classification is pretty hard to do as you can see. As a result, derechos are fairly rare, usually occurring only once a year somewhere in the US. Specifically for Illinois, we usually see one derecho come through the state every other year.
The last time we saw one in the state was August 10th of 2020, so based on the averages, one may pass through the state this severe weather season.
The August 10th derecho is notable for many reasons, mainly for being the costliest thunderstorm in US history. It started out as a cluster of storms in southeastern South Dakota and northeastern and quickly gained strength as it crossed Iowa. It is in Iowa that it had the strongest wind gusts of 140 mph near Cedar Rapids.
NOAA estimates that this was a 11 billion dollar event, not only due to the structural and infrastructure damage, but also destroying mature crops.
Here in Central Illinois, we were spared from most of the damage since the strongest part of the derecho was in northern Illinois. There were many tornado reports in the Chicagoland area. That being said, we did have a 75 mph gust reported near Danville by the Vermilion County Emergency Manager and some minor tree damage across our counties.