The July 2011 Iowa-Illinois-Michigan-Ohio Derecho was a derecho event that happened on July 11, 2011 in the states of Iowa, Illinois, Michigan, and Ohio.
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On the morning of July 11, 2011, a powerful long-lasting straight-line windstorm, known as a derecho, developed over central Iowa and carved a path of extensive damage across east central Iowa. The storm first took shape as a cluster of low end severe storms over southern Nebraska during the late afternoon of July 10. The system continued northeastward and entered western Iowa at 1:00am, still only as a marginal line of severe storms. As the system passed through the northern Des Moines metro area at 3:30am, it interacted with an outflow boundary from the storms to the north and rapidly intensified as it accelerated eastward. Over the next hour and a half the storm plowed eastward through Story, Marshall, and Tama Counties, blasting the area with winds of up to 105 mph. The storm continued to track eastward, plowing through eastern Iowa and the southern Great Lakes region before dissipating in West Virginia in the mid-afternoon. Thousands of trees were downed in western Iowa alone, and numerous structures were either damaged or destroyed. The peak winds were estimated to be in the range of 130 miles per hour (210 km/h), equal to a Category 4 hurricane on the Saffir-Simpson hurricane scale
Shortly after this complex, a major heat wave would affect the eastern half of the continent, with another significant derecho event occurring on July 17, 2011 over Ontario, Quebec and northern New England.
In Iowa
The derecho started around 3:20 Monday morning just north of Des Moines and ended around 18 hours later somewhere in the Atlantic Ocean. In Iowa, wind speeds topped out at 130 miles per hour around Vinton and Garrison.
In Garrison, Iowa, straight line wind of 110 to 130 mph produced widespread damage to structures. Many roofs were partially or fully removed and the walls of some buildings collapsed, including the fire station and town library. Nearly every tree was significantly damaged or snapped off . Power outages were widespread.
At Vinton, Iowa, straight line wind in excess of 110 miles per hour (180 km/h) caused damage to numerous roof structures and some exterior walls, including a downtown apartment building. Nearly every tree was significantly damaged or snapped off. The Iowa Braille and Sight Saving School was also severely damaged. The local high school was utilized as an emergency shelter for two or three days after the storm until power was restored throughout the community. The building which housed several newspapers produced locally was damaged and forced the operation to be relocated for several months. In the Dudgeon Lake Wildlife Management Area across the Cedar River north of Vinton, extensive damage was left in the wake of the Derecho with more than half of the trees badly damaged. Some estimated it would take 500 years for the area to fully recover.
In Illinois
During the morning hours of July 11, 2011, a line of severe thunderstorms moved very quickly across northern Illinois and southern Lake Michigan, producing widespread wind damage. In addition, the strong thunderstorm winds with this system generated a classic seiche event on the lake. A seiche is a situation where lake water ahead of the storms is piled up along the downwind shore (in this case Indiana and Michigan) and then sloshes back and forth across the lake for several hours.
In Chicago, this derecho left more than 860,000 people without electricity.