A derecho (/dəˈreɪtʃoʊ/, from Spanish: derecho[deˈɾetʃo], “straight” as in direction) is a widespread, long-lived, straight-line wind storm that is associated with a fast-moving group of severe thunderstorms known as a mesoscale convective system.
Most of the world’s derechos occur in the U.S. While rare, derechos tend to form in the Midwest and track to the east. The Southern Great Lakes Derecho of May 31, 1998 is the most famous derecho to hit West Michigan.
The fastest estimated wind from the Iowa Derecho was 140 mph – equivalent to an EF3 tornado. The fastest measured wind gust was 126 mp;h at Atkins. The town of Midway recorded a gust to 112 mph. With wind gusts that strong, anemometers can fail, mainly because they are impacted with debris.
This is the WMT radio tower, toppled by winds estimated at 130 mph.
The storm blew down an estimated 6.1 million acres of corn and soybeans – roughly 20% of the total crop of the state of Iowa. Half of the tree canopy of Cedar Rapids IA was destroyed.
A total of 1.4 million customers lost power during the sorm, including 98% of the city of Cedar Rapids. Three days after the storm, there were still 300,000 homes without power in IA and IL. In the Cedar Rapids area, 3,400 utility poles had to be replaced and power wasn’t fully restored until 2 weeks after the storm.
This is the storm approaching Sugar Grove, Illinois. The storm’s estimated cost was 11 Billion Dollars! That’s the highest dollar damage total for any severe thunderstorm event in world history. An interesting note – more people were injured after the storm than during the storm.
Here’s two satellite images showing the loss of power due to the storm across Iowa. The derecho traveled 770 miles at an average speed of 55 mph.
The storm system spawned 25 tornadoes, most of them in N. Illinois. While the tornadoes caused damage, they were all rated EF0 or EF1, so damage was similar to the strong thunderstorm winds. As I wrote earlier, the strongest straight-line thunderstorm winds produced EF3 damage.
While the greatest damage was across Iowa and Illinois, the storms did move across N. Indiana and did produce damage in SW Michigan from winds up to 60-70 mph. 500 trees were toppled in Chicago parks.
By Sept. 28, the city of Cedar Rapids hauled away 53,598 truckloads of debris (probably as many loads were privately hauled away). There was an uptick in allergies from the rotting vegetation and debris.
There are lots of videos of the storm on YouTube. I have watched most of them – the hour long video I’ve watched twice. Watching the storms on YouTube is safer than chasing a storm like this.
Remember, when we have the threat of strong winds – tornado or thunderstorm – move to the lowest floor, stay away from windows, try to put more than one wall between you and the storm, stay away from windows, take down hanging baskets, turn trampolines upside-down, close your garage door, don’t park your vehicle under a tree if possible, charge your phone, try to find a way to keep your pets calm and watch out for stay animals, pets and livestock, after the storm. Fences are blown over and animals can be in an agitated state.
After the storm, stay away from downed power lines, be careful cleaning debris, be on guard against falling.