As Illinois experiences a third consecutive year of record-breaking rainfall stretching from Chicago to Cairo, researchers at the Illinois State Water Survey (ISWS) have updated the publication that provides Illinois’ standards for expected extreme storms, known as Bulletin 75.

“The heavy precipitation in Illinois and the Midwest presents a significant challenge in terms of stormwater management,” hydrologist Momcilo Markus said. “To address the problem effectively, more accurate predictions of future rainfall intensity and frequency are critical and require up-to-date assessments of current climate conditions and model-generated data for future horizons.”

Markus and former State Climatologist Jim Angel co-authored the report using climate modeling projections, which indicate that not only is rainfall expected to increase, but that the amount of the increase will grow.

Kay Whitlock, vice president at Christopher B. Burke Engineering, Ltd., has been managing water resources, stormwater, and flood control projects in Illinois for over 40 years, and she has been working with data from ISWS for over three decades.

“The data that Bulletin 75 provides is pervasive across everything we do, and we are using it to tell a better story,” Whitlock said. “It’s a great tool to explain to elected officials everything we do, and the science collected and analyzed by the researchers at PRI is a big part of what we do.”

Effective January 2020, the Illinois Department of Natural Resources Office of Water Resources and District 1 of the Illinois Department of Transportation began requiring Bulletin 75 rainfall for newly initiated projects. The Federal Emergency Management Agency will also accept Bulletin 75 rainfall data for projects and map revisions. Illinois counties and communities are expected to update ordinances that will adopt the new Bulletin 75 standards for building structures based according to a predefined magnitude and duration of storms.

“Working with our clients and government agencies on major projects across the state, such as Illinois highway projects, the Illinois Tollway, and renovations to O’Hare airport, we work with municipalities and regulatory agencies that require all hydrological analysis and frameworks to be built according to the standards of the newest version of Bulletin 75,” Whitlock said.

The updated publication, supported by the Illinois Department of Commerce and Economic Opportunity and the U.S. Department of Housing and Urban Development, used an additional 34 years of data, which show that the number of storms in Illinois producing over 2 inches of rain has nearly doubled over the past century. During that span, the statewide average annual precipitation has increased 11 percent and temperatures have risen by 1.2 degrees.

“Climate modeling projections indicate that not only have we seen an increase in rainfall, but that this will increase in the future,” said Markus, “The 100-year, 24-hour rainfall is about 1-inch higher than the old one in the Chicago area – this is 13% higher.”

A 24-hour 100-year storm event is the amount of rain that should accumulate in a 24-hour period for a storm with a 1-in-100 (or 1%) chance of occurring in a given year. Earlier versions of Bulletin 75 state that a 24-hour 100-year storm event will result in approximately 7.58 inches of precipitation.

The new data suggest Illinoisans should expect nearly an additional inch of rainfall for a 24-hour 100-year storm event (8.57” instead of 7.58”). 

The Urban Flooding Awareness Act recommends that rainfall frequency distribution data be updated every 15 years. 

ISWS is also collaborating with the National Oceanic and Atmospheric Administration (NOAA) to update Atlas 14, which serves as the benchmark for precipitation frequency values across the United States.

This work will provide updated methodology for precipitation estimates for the Northeastern states, where statistical analyses and observational data have detected increasing trends in the intensity and frequency of heavy precipitation.

Markus notes the observed and projected increases in heavy precipitation happening in the Northeast are comparable to the increasing rainfall trends happening in Illinois.

The Illinois State Water Survey is a division of the Prairie Research Institute at the University of Illinois at Urbana-Champaign.