Published: July 20,2017
A cluster of severe thunderstorms Wednesday and Wednesday night in the northern Plains and upper Midwest was a low-end derecho, with reports of wind damage and high winds spanning parts of six states.
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Some 140 reports of strong thunderstorm wind gusts or wind damage were received by local National Weather Service offices from northern South Dakota to northwest Indiana Wednesday.
Preliminary severe weather reports on July 19, 2017. The yellow arrow roughly highlights the path of the derecho from South Dakota to northwest Indiana.This almost-continuous swath roughly 750 miles long lasted more than 14 hours from the first severe wind report in Corson County, South Dakota, around 8:30 a.m. CDT to the final wind report around 10:45 p.m. CDT in Porter County, Indiana.
Radar history of the July 19, 2017, derecho in the Midwest from 8:00 a.m. CDT to 10:00 p.m. CDT.
If you like a different view of this, University of Wisconsin research meteorologist Scott Bachmeier put together a spectacular loop of infrared satellite imagery with severe reports overlaid.
This satisfied the criteria to be dubbed a derecho, according to a 2005 study by Walker Ashley and Thomas Mote.
From the Spanish word for "straight," these convective windstorms leave swaths of wind damage at least 250 miles long and are most common in the summer months from the upper Midwest to the Ohio Valley.
(MORE: Summer Derechos Have a Favored Corridor)
While not a necessary condition, derechos often contain several reports of winds over 75 mph, capable of downing numerous trees, often toppling onto power lines, cutting power to hundreds of thousands – sometimes during a sweltering summer stretch.
Damage HighlightsWhat eventually became a derecho began in South Dakota with some eye-popping wind damage.
A wind gust of 100 mph was measured by a sensor in Hyde County. Several grain bins were destroyed, according to county law enforcement.
This cell then roared through Huron, South Dakota, with a wind gust to 83 mph accompanied by hail as big as tennis balls.
Near the South Dakota-Minnesota state line, a barn and grain bin were destroyed northwest of Lake Benton, and damage, possibly from an embedded tornado, was reported south of Marshall, Minnesota.
After that, wind damage became more widespread as the thunderstorm cluster developed several pronounced "bow echoes."
Widespread wind damage occurred in northeast Iowa early Wednesday evening. Wind gusts up to 70 mph downed numerous trees and power lines, damaged homes and flattened corn fields.
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The derecho then tracked into southwest Wisconsin, where a hangar at the airport in Prairie du Chien was damaged.
A 70-mph wind gust was measured at the Boone County Fairgrounds east of Rockford, Illinois, just before 9 p.m.
Weakening a bit, this cluster of thunderstorms then pushed through Chicagoland about an hour later, with a couple reports of tree damage on the west side of the metro area.
If that wasn't enough, a second thunderstorm cluster behind the derecho dumped torrential rain on already saturated ground in parts of southeast Minnesota and western Wisconsin overnight, triggering destructive flash flooding in Arcadia, Wisconsin that swamped roads, triggered mudslides and prompted evacuations.
How This Derecho Measures UpThis wasn't a particularly intense derecho – not close to the magnitude of the June 29, 2012 event which knocked out power to millions in the Ohio Valley and mid-Atlantic states. If derechos had their own Saffir-Simpson scale like hurricanes, this might be a Category 1.
This event seemed to be a light version of another derecho 34 years prior, which tracked farther north and was more organized than Wednesday's event, as detailed by NOAA's Storm Prediction Center.
This derecho is a good reminder why you should never ignore severe thunderstorm warnings and would be wise to take shelter when you hear a severe thunderstorm warning associated with a squall line as you would a tornado.
Always have multiple methods to receive National Weather Service watches and warnings – for example, The Weather Channel or Weather Underground smartphone app with alerts turned on, or NOAA weather radio.
Make sure the alert or alarm is turned on while you sleep so you can be awakened if severe weather, including a derecho, is a threat overnight.
Jonathan Erdman is a senior meteorologist at weather.com and has been an incurable weather geek since a tornado narrowly missed his childhood home in Wisconsin at age 7. Follow him on Facebook and Twitter.
MORE: June 2012 Derecho Damage (PHOTOS)
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