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Antecedent conditions and precipitation summary, July 24, 2010

July 26, 2010
Monthly precipitation through July 25 at Carol Stream 0.3 SSE

Monthly precipitation through July 25 at Carol Stream 0.3 SSE

Antecedent conditions refer to the state of the Klein Creek watershed prior to the heavy rainfall event. The above information (though not in the handy monthly chart form) is available through CoCoRaHS at this link: http://www.cocorahs.org/ViewData/

This station is IL-DP-42. Station IL-DP-11 reports from the northwest side of Armstrong Park.

In the 10 days prior to the July 24 event, only 0.37″ of rain fell, compared to 3.18″ in the 10 days prior to the September 13-14, 2008 event. This is a major point; the more rain falls prior to a heavy rain event, the less rain that can be absorbed during the heavy rain event itself.

This dryness left the top 6-12″ of the ground bone-dry and able to absorb some resulting runoff from the coming storm.

The Event

Radar Loop for July 24, 2010 (click to see loop)

At 11:30 PM, the Storm Prediction Center in Norman Oklahoma issued mesoscale discussion (MD) 1445, which also contained a graphic of the general meteorological situation as it was unfolding. The meteorological details of this event will be covered in a later post, but the MD itself warned of 1-2″/hr. rainfall rates through 09z (4:00 AM local time):

MD 1445

A later mesoscale discussion indicated the threat was to be extended through 12z (7:00 AM local time).

Ground truth verified these MDs. Station IL-DP-42 reported storm total rainfall amounts of 3.92″ at 1:15 AM, 5.24″ at 3:45 AM and 7.19″ by 6:30 AM.

At 6:00 AM, a derecho swept to the north of the area, bringing extremely heavy rain but also a conclusion to the relentless rounds of thunderstorms observed at the area. After this point, only light rain was reported in the Carol Stream area.

Precipitation totals

Radar-estimated storm total rainfall, July 24, 2010

The image above depicts estimated rainfall amounts for the area through about 8:00 AM local time on July 24. A wide swath of 6 to 8″ totals was observed from a 20-mile-wide line from DeKalb east-southeast to downtown Chicago. This graphic is good for a general depiction of areawide totals, but does not contain the detail necessary to further analyze the event. Click here to create your own map.

Click to enlarge

This image represents CoCoRaHS totals for the period through approximately 9:30 AM on July 24th. At that time, Carol Stream 0.3 SSE reported a total of 7.28″ of rain. Glen Ellyn reported 8.20″ of rain, while Hanover Park reported 7.62″ of rain. This information is accessible through this link: http://www.cocorahs.org/Maps/ViewMap.aspx

This amount of precipitation caused widespread, massive flooding throughout the Klein Creek floodplain. Analysis and images of the flooding will be performed in the next post.

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