* WHAT...Heat index values 100 to 104 expected. Overnight heat indices in the middle 70s. * WHERE...Portions of northern Indiana and northwest Ohio. * WHEN...From 2 PM EDT /1 PM CDT/ Tuesday to 8 PM EDT /7 PM CDT/ Wednesday. * IMPACTS...Hot temperatures and high humidity may cause heat illnesses. * ADDITIONAL DETAILS...A period of dangerous heat and humidity is in store through at least Wednesday evening, with minimal relief Tuesday night. Chances for showers and thunderstorms should remain north of the advisory area.