A Dense Fog Advisory was also issued Thursday until 9 a.m. for Boone, DeKalb, Will, Ford, Grundy, Iroquois, Kane, Kankakee, Kendall, LaSalle, southern Cook counties.
For Indiana, Jasper, Lake, Newtown and Porter counties the Dense Fog Advisory is also in effect until 9 a.m.
ABC7 meteorologists said heavy rain is developing for Thursday afternoon and evening.
The Flood Watch is in effect through Friday at 10 a.m. for Ogle, Lee, DeKalb, La Salle, Kendall, Grundy, Kankakee, Livingston, Iroquois, Ford, northern Will; southern Will; eastern Will counties.
Also in Indiana, the Flood Watch is in effect for Jasper, Newton and Benton counties until Friday at 10 a.m.