Using raingage data, Illinois State Water Survey Senior Scientist Stan Changnon has analyzed an exceptionally heavy rainstorm across much of Chicago that dumped 4.78 inches of rain in the Loop within a 3-hour period on August 2.

The Illinois State Water Survey, a division of the Illinois Department of Natural Resources, operates this network of 25 recording raingages distributed across Cook County. Funds from the U.S. Army Corps of Engineers support network operation and data collection and analysis supervised by research meteorologist Nancy Westcott. Gages recorded rainfall amounts for each 10-minute interval during the storm, data used by Changnon in evaluating the storm.

Meteorologists rate heavy rains based on their expected return period. The longer the return period, the rarer the event. Assessment of the heaviest one-hour amounts in the August 2 storm revealed just how rare these storms are.

"Raingages in northern Cook County had one-hour values of 2 to 2.3 inches, amounts expected to occur only once every 10 to 15 years. Between 6:45 and 7:45 a.m., 3.5 inches fell in the Loop--a once-in-75-year amount," says Changnon.

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Storm impacts in the city of Chicago were extensive, and Governor George Ryan made a State disaster declaration for both Chicago and Cook County. After the first hour of rain, the 2.23-billion-gallon capacity of the sewer system and Deep Tunnel were filled. Flooded underpasses closed most expressways. Subway and train systems were rerouted to avoid flooded tunnels. Airport delays exceeded two hours. More than 56,000 customers lost power. Water backed up into more than 300,000 basements as combined surface and sewer drains could not sustain the volume of water. Sewer water then was released into the Chicago River and Lake Michigan.

"One-hour amounts in southern Cook County were less, ranging from once-in-3-year to once-in-20-year values. Near record one-hour amounts fell between 6:30 and 7:30 a.m. in northern Cook County, from 7 to 8 a.m. in central sections, and from 8 to 9 a.m. in southern Cook County," continues Changnon.

What makes the rainfall unique is that most of the rain at any point fell in three hours or less. "The map of total rainfall from the storm reveals several salient features. Peak rainfall centered in the Loop, nearly 5 inches over a 3-hour period--an amount expected to occur only once in 80 years. Rainfall in other parts of central Cook County was rated as once in 40- to 50-year amounts. Rainfall in excess of 3 inches, amounts expected to occur only once in 15 years, fell along the lake shore and extended inland westward to La Grange and well south of Midway Airport. Amounts were less than an inch in extreme western Cook County, amounts expected to occur every 3 or 4 months," concludes Changnon.

Just 12 days earlier on July 21, similar heavy, short-duration rains occurred in southwestern Cook County. The Water Survey's raingage near Orland Park recorded 4.57 inches of rain in one hour, an amount expected to occur only once in 125 years. The Chicago Heights area received 2.9 inches and Lemont received 2.5 inches, amounts expected once in 40 years and once in 25 years, respectively.

(Staff from the Midwestern Regional Climate Center contributed to this story.)