National Weather Service United States Department of Commerce

Basic Facts and Statistics

  • At the time it ranked as the second most costly weather disaster in Illinois behind the 1993 flood. Damages totaled $600-700 million. Recovery also lasted for over a year after the flooding occurred.
  • 16.94 inches of rain fell in Aurora setting a statewide record for the greatest rainfall for a single 24-hr period.
  • This impressive rainfall event was the result of a Mesoscale Convective System (MCS) that developed in the evening of July 17 and moved slowly over northern Illinois.
  • In Illinois 6 people were killed from this event. 2 people were killed in Iowa due to the MCS the night prior to the one over Illinois.
  • Prior to the MCS formation, severe weather during the afternoon of the 17th produced large hail, damaging lightning, and three tornadoes.
  • Only about 1% of the people who owned homes that experienced flood damage had flood insurance.

Record Rainstorm and Flood of July 17-18, 1996
By Jim Angel, Illinois State Climatologist

 

The rainstorm on July 17-18, 1996, produced several rainfall records and was the second most costly weather disaster in Illinois behind the 1993 flood. The 16.94 inches recorded at Aurora still stands as the statewide record for the most rain from a single 24-hour period. The 10.99 inches on the west side of the Chicago metro area was the most recorded in the Chicago urban area. Just as impressive as the point values was the size of the area covered by heavy rainfall. It was estimated that 16.3 inches fell over the wettest 100 square mile area of the storm, 12.6 inches over the wettest 1000 square mile area, and 5.2 inches over the wettest 10,000 square mile area. Another way to look at it is that the an area of 1350 square miles exceeded the expected 100-year, 24-hour storm while 4650 square miles exceeded the expected 10-year, 24-hour storm for northeast Illinois.

The widespread heavy rains led to excessive flooding. Damage estimates were on the order of 600-700 million dollars (in 1996 dollars). FEMA estimated that more than 35000 residences were flooded. By every measure, the July 17-18 storm was a truly remarkable event.

Some 201 NWS gages, a vast majority of them cooperative observers, as well as 278 gages from other networks were used to define the area of the storm. In addition, data from long-term cooperative observer sites were used to calculate the design values such as the 100-year, 24-hour storm.

More information on the July 17-18 storm can be found in this Water Survey publication: Angel, James R., Stanley A. Changnon, David Changnon, Floyd A. Huff, Paul Merzlock, Steven R. Silberberg, and Nancy E. Westcott, 1997 Illinois State Water Survey, Champaign, IL

The Meteorology
by Alex Morrison

July 16

On the morning of July 16, 1996, it became apparent that a severe weather event with heavy rainfall was imminent.  In the few weeks prior, the area had been dry and it would require a lot of rainfall over a large area to produce flash flooding.  At the time forecasters were much more concerned severe storms as the primary threat. 

On the night of the 16th, there was also another Mesoscale Convective System that brought rain to most of Iowa and a few parts of northern Illinois. This rainfall also hinted at the possibility for storm development on the 17th and ultimately the evaporated water from this rainstorm became the initial moisture used for storm development the next day.

July 17

By the morning of the 17th, awareness of the possibility of a major convective event was heightened. Models consistently showed severe weather and heavy rainfall development in the afternoon and into the evening. A warm front was analyzed from southeastern Nebraska to southern Illinois moving northeast. This front became the main forcing for severe weather later in the day. Easterly winds coming off of the lake brought moist and relatively cooler air into northern Illinois causing the warm front to be held farther west until later in the afternoon.

The warm front continued to become more defined as it slowly moved northward. Dew points north of the warm front drastically increased due to an increase in moisture. Pressure falls were also seen in eastern Iowa and far northwestern Illinois. Wind profiler data showed significant moisture convergence over northern Illinois. All of these factors caused forecasters to become considerably more concerned for excessive rainfall and severe weather later that day into the evening. A Flash Flood Watch was issued at 4PM indicating the potential for heavy rainfall and flooding across northern Illinois.

Throughout the early afternoon thunderstorms began to develop and increase in coverage across northern Illinois. However, these storms did not meet the criteria to be considered severe storms. As the day went on, thunderstorms continued to develop and increase in intensity. Severe storms were not seen until 2 PM when one storm that developed south of Chicago produced a tornado.

 At about 4 PM, infrared satellite imagery suggested a new phase of the rainstorm. Prior to this, storms were shown to have relatively warm cloud tops, but after this time satellite indicated much cooler tops meaning deeper, greater water-content storms and that storms were becoming more west-to-east oriented. This was of concern because the storms were about to train over some locations with continuous heavy rainfall.  Another important observation was the small development of storms in eastern Iowa and western Illinois. This small development hinted of future system evolution during the evening hours that ultimately become an MCS.

Into the evening hours,  strong southwest winds just off the surface known as a low level jet had strengthened and would be key in replenishing very moist air into the storms over northern Illinois. A sounding from 6PM indicated the necessary ingredients for MCS development of warm moist air, a strong low level jet, large instability, and strong helicity.

The continued strength of the low level jet throughout the night helped fuel this storm with plenty of moisture and warm air that caused the continued development of heavy rainstorms over northern Illinois up through the early morning hours of July 18. Finally, just a few hours before sunrise the MCS began to weaken and progress eastward, ending the devastating rainstorm.

July 1996 Rainfall plot

  • Channahon, IL
  • Interstate 55 flooding in Will County
  • Near Caton Farm Rd in Plainfield, IL
  • Near Caton Farm Rd in Plainfield, IL
  • Shorewood, IL
  • Shorewood, IL
  • Shorewood, IL
  • Shorewood, IL

Flooded Communities  Hydrograph