Temperatures are expected to climb into the 90s Thursday with high humidity. The heat index could reach 105 degrees.
The heat advisory, which was issued at 10 a.m. Thursday, includes DeKalb, Kane, DuPage, Cook, LaSalle, Kendall, Grundy, Will and Kankakee counties in Illinois and Lake County in Indiana.
A heat advisory is called when high temperatures and high humidity combine. To cope with the heat, people are encouraged to drink water, wear light and loose clothing, and reschedule any strenuous activities to early morning or evening.
"You can already feel how humid it is starting to get. Most of what we have now in the area is very light rain," ABC7's Tracy Butler said.
That light rain will give way to strong storms Thursday evening. Excessive rainfall is expected -- possibly as heavy 1 to 3 inches of rain per hour during the height of the storm.
Because the area has seen so much rain already, flooding is expected, especially in areas along and north of Interstate 88 and Interstate 290. A flash flood watch is in effect for Boone, Cook, DeKalb, DuPage, Kane, Kendall, Lake, Lee, McHenry, Ogle, Will and Winnebago counties in Illinois. In Indiana, Lake and Porter counties are also under a flash flood watch.
This month has been the wettest July ever -- with 9.8 inches of rain measured at O'Hare International Airport -- and the ninth wettest month overall.
A flood advisory in effect until late Saturday night along the east branch of the DuPage River at Bolingbrook. The DuPage River is expected to rise to almost 19.1 feet by Friday evening.