Posted at 11:45 AM ET, 07/11/2011

Chicago blasted by violent line of thunderstorms, known as derecho

Radar showing storms approach Chicago Monday morning around 8 a.m. local time.

6:15 p.m. UPDATE: The Chicago Sun-Times reports the storm was “largest ever” for ComEd with 817,000 customers losing power at one point.

From 11:45 a.m.: A ferocious line of thunderstorms with winds to hurricane strength tore through Chicago this morning, knocking out power to more than 660,000 residents. The Weather Channel reports that power provider Commonwealth Edison is saying this could be one of the top five outages in their system’s history.

Widespread reports of wind gusts to 70 to 80 mph were observed as the storms raked the area. Midway airported reported a gust to 75 mph and O’Hare gusted to 63 mph. WeatherBug indicates 17 of its local observing stations registered winds of at least 60 mph.

The line of storms contained all of the characteristics of a derecho, defined as a widespread, long-lived windstorm associated with a band of rapidly moving showers and storms.

High winds at about 18,000 feet energized the fast-moving storms that ripped through Chicago Monday morning. (
The derecho was energized by very strong high altitude winds of 60-70 mph and abundant low level moisture. Its impacts extended well beyond Chicago. AccuWeather’s Henry Margusity tweeted as of 11 a.m. that the storms have traveled 425 miles, produced almost 200 wind damage reports, and caused more than a million power outages. As of 11:30 a.m., the storms stretched from around Lansing, Michigan to Fort Wayne, Indiana. The storms are likely to affect Detroit, Ann Arbor and Toledo Ohio through early this afternoon with the potential for damaging winds.

As the derecho surged eastward, it piled water up along the eastern shores of Lake Michigan along in Michigan and Indiana. That water is now forecast to reflect (or slosh) back toward the Illinois shoreline. This phenomenon, known as a seiche, has prompted a lakeshore flood warning along the Illinois shoreline of Lake Michigan, for an increase in water levels of two or more feet through 1 p.m. central time or noon eastern. Meteorologist Brad Panovich has a great blog about this phenomenon.

Watch a video of the storm moving through the city, blowing lounge chairs into a rooftop swimming pool:

This is the third significant severe thunderstorm outbreak in the Chicago area in the last month. (See recap of outbreak from last week)

Additional Reading: 660,000 without power from fast-moving storms ( Chicago Tribune )

Photo gallery from Tom Skilling’s Facebook page with damage pics

By  |  11:45 AM ET, 07/11/2011

Categories:  U.S. Weather, Latest

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