* WHAT...For the Heat Advisory, heat index values up to 105 expected. For the Extreme Heat Warning, dangerously hot conditions with heat index values of 105 to 110 expected. * WHERE...Portions of southern Illinois, southwest Indiana, western Kentucky, and southeast Missouri. * WHEN...For the Heat Advisory, until 8 PM CDT /9 PM EDT/ this evening. For the Extreme Heat Warning, from 11 AM CDT /noon EDT/ Monday to 8 PM CDT /9 PM EDT/ Thursday. * IMPACTS...Heat related illnesses increase significantly during extreme heat and high humidity events. Hot temperatures and high humidity may cause heat illnesses. * ADDITIONAL DETAILS...An extended duration heatwave will impact the region through at least Thursday. Overnight lows will only fall into the mid 70s, providing little relief.