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PostPosted: Mon Jul 11, 2011 11:36 pm 
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7-11-2011

Fast-moving storms knock out power to 852,000

Thunderstorms packing winds of 75 mph knocked out power to about 852,000 Commonwealth Edison customers this morning, the biggest outage from a storm in at least 13 years.

ComEd spokesperson Tony Hernandez told WGN radio's John Williams this afternoon that "this is going to be one for the record books."

The company warned it could take days to restore all power. It had 480 crews out working, with more being called in from neighboring states.

Several semi-tractor trucks were reported knocked over by winds, including on 65th Street at Menard Avenue inChicago and on Interstate 94 at Illinois Route 176 in Greek Oaks in Lake County.

In Waukegan, about 100 residents had to leave an apartment complex after winds ripped off roofs from two buildings of the three-story complex at Golf Road and Pine Street.

The storms moved in at 70 mph. Midway Airport clocked winds of 75 mph and O'Hare Airport saw 63 mph winds mph as 65 mph winds hit Naperville.

Chicago National Weather service report

July 11th Severe Storms and Seiche

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 downstream shore (in this case Indiana and Michigan) and then sloshes back and forth across the lake for several hours. The danger from a seiche comes from rapidly rising and falling lake levels, which can damage watercraft along the shoreline, sweep people off piers and breakwaters, and pull swimmers far away from shore.

The storms responsible for this event developed late Sunday into early Monday across Nebraska and western Iowa. Overnight they produced extensive wind damage across much of Iowa. By 6:00 AM CDT as they moved into Illinois, a very moist unstable airmass (MLCAPE of 2000-4000 J/Kg) with considerable deep layer wind shear (40-50 kts) was in place ahead of the storms. The Craven/Brooks Significant Severe Parameter, which represents the contribution from both MLCAPE and deep layer shear, was showing values as high as 60,000 m3/s3 (while normally values around 20,000 are considered sufficient for significant severe storms).

2011 July 11 -- 11Z Craven Brooks SigSvr Parameter

While the wind shear was sufficient to support organized severe storms, the westerly unidirectional nature of the winds was more conducive to the development of strong straight line wind damage versus tornadoes. Despite the lack of directional shear, or actually because of it, the strong mid and upper-level winds were able to mix down to the surface and produce a wide swath of 60-80 mph measurements -- plus the associated damage.


2011 July 11 -- 13Z Mean wind from 850-300mb

The effects of the seiche with this system were most readily visible from the water level measurements at the Calumet Harbor data collection platform. The following image shows the weather conditions prior to, during, and after the passage of the line of thunderstorms. The top graph shows water levels starting to fluctuate considerably soon after the line passed, eventually with over 2 feet of difference from peak to trough.


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