Bulletin 70 (http://www.isws.illinois.edu/atmos/statecli/RF/download.htm) has an entire section (section 4) on "URBAN EFFECTS ON FREQUENCY DISTRIBUTIONS" and specifically "The Chicago Effect" which appears to show that the thresholds for n-year storms in Chicago could be different from the thresholds in the Northeast Climatic section as a whole.
Bulletin 70 (http://www.isws.illinois.edu/atmos/statecli/RF/download.htm) has an entire section (section 4) on "URBAN EFFECTS ON FREQUENCY DISTRIBUTIONS" and specifically "The Chicago Effect" which appears to show that the thresholds for n-year storms in Chicago could be different from the thresholds in the Northeast Climatic section as a whole.