http://blog.chicagoweathercenter.com/severe-blog/2013/05/20/seasons-first-organized-severe-weather-threat-expected-later-today/

Cool temperatures and the absence of high humidity have suppressed severe thunderstorm activity across much of the Midwest this Spring.  Locally, the quiet severe weather season is expected to come to an abrupt end late this afternoon and this evening as clusters of strong to severe thunderstorms rake across much of Illinois, northern Indiana and southern Wisconsin.

Timing

Storms could fire anytime this afternoon, however the highest probability will be between 6PM and 1AM.