Storm frequency estimates and their temporal distributions are important in determining estimatesof runoff or peak flow rates in many engineering and hydrological problems. Illinois State Water SurveyBulletin 70 has been serving as the design rainfall standard in Illinois since its publication in 1989. AlthoughBulletin 70 represented the best available data at the time of its publication, the standards needed to be reevaluatedand updated after more than three decades and with the growing evidence of the nonstationary nature ofheavy precipitation. The trends in heavy precipitation in Illinois prompted the creation of a new frequency studynamed Bulletin 75, providing precipitation frequencies for event durations ranging from 5 min to 10 days andfor recurrence intervals ranging from 2 months to 500 years. The results are presented for the same 10 geographicsections in Illinois as in Bulletin 70 to maintain the continuity of hydrologic studies and compatibilitywith regulations. The primary goal of this paper is to outline some of the key methodological issues and challenges,to compare the results with the previous sources, and to highlight the effects of the changing precipitationstandards on the development of amendments to existing ordinances. Lake County in Illinois, as one of themost affected urban areas with the highest change in heavy precipitation, was selected to illustrate issuesrelated to the application of modified precipitation standards.
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