Case Review: 1718 July 1996 Summary of the Northeastern Illinois Record Rainfall and Flash Flood Eve

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Case Review 17-18 July 1996Summary of the
Northeastern Illinois Record Rainfall and Flash
Flood Event.

• Excessive Rainfall Struck the western and
  northwestern suburbs of Chicago between 0000 and
  1000 UTC 18 July 1996. Flash flooding in these
  urbanized basins and sub-basins resulted in 8
  fatalities, flooded more than 35,000 homes, and
  caused more than 6 Million in private, public
  and agricultural damages.
• An Illinois all-time 24-h rainfall record of 430
  mm (16.94 inches) fell in Aurora Illinois. The
  peak rainfall occurred between 0800 and 1000 UTC.
  A number of streams were severely impacted,
  including the DuPage, Fox, and Illinois Rivers,
  and Blackberry Creek.

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Case Review 17-18 July 1996Summary of the
Northeastern Illinois Record Rainfall and Flash
Flood Event.

• The setup was a rather classic frontal event,
  although with time it evolved into more of a
  mesohigh system. Regardless of what type it was,
  the important consideration was the strong and
  very moist low-level flow into a quasi stationary
  boundary supported rapid regeneration of storm
  echoes in a common area. This regeneration
  obtained a balance with the movement of
  individual storm cells to the east-southeast
  resulting in a narrowly-defined area a repeated
  intense rainfall just on the cool side of the
  mesohigh boundary. Hand analyses indicate that
  the mesohigh became more well-defined just prior
  to the peak of the storm, but indication of the
  boundary existed as early as 0000 UTC.

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Case Review 17-18 July 1996Summary of the
Northeastern Illinois Record Rainfall and Flash
Flood Event.

• The Chicago radar (KLOT) was experiencing very
  intense precipitation on the radome in the latter
  part of the storm. It was at this time that the
  discrepancy between KLOT and KDVN was greatest,
  most likely due to attenuation of the KLOT radar
  beam. Nevertheless, the radar provided excellent
  temporal and spatial detail of this event.

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Radar Storm Total from KDVN (Davenport) 18 July
1996 1400 UTC
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Radar Storm Total from KLOT (Chicago)18 July 1996
1400 UTC
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Illinois State Water Survey
Isohyet Analysis as of 1200 UTC 18 July 1996
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Illinois State Water Survey
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Illinois State Water Survey
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Urban Communities affected by severe flooding on
17-18 July 1996
Illinois State Water Survey
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Case Review 17-18 July 1996Summary of the
Northeastern Illinois Record Rainfall and Flash
Flood Event.

• Rainfall Duration enhanced do due to a common
  regeneration area and subsequent movement of
  echoes across the same narrow corridor
  (training).
• Rainfall Intensity was also enhanced in the very
  moist atmosphere (high theta-e source air) and
  high precipitable water. The low level boundary
  also resulted in extra low-level lift to support
  collision coalescence processes.
• Hydrologic Response the extended duration of
  intense rainfall bursts saturated the soil
  layers, and the urban hydrology further enhanced
  the tendency for very rapid runoff.