We have seen plenty of cloud cover across the area over the last several days, but no rainfall has come from it. That will change beginning Wednesday morning as a band of scattered rain will move through between 8am and 2pm. The coverage won't be impressive, nor is heavy rain in the forecast, but Wednesday's shower threat is a sign of things to come. Thursday morning through Friday morning is the time frame to watch for heavier, more concentrated rain and thunderstorms. By Thursday evening, forecast models are suggesting that rainfall totals from Rockford points north and west could be over 1-2". That doesn't include the rain we are expecting Thursday night into Friday morning.
Flooding doesn't look like a major concern, especially considering the fact that September ended below average in the rainfall department. However, when you get hefty rainfall totals in a little period of time, there is always a possibility of seeing at least some high water, ponding on roadways, and incidences of isolated flooding. The severe weather threat is extremely low for the northern Illinois area, but some thunder and lightning are certainly possible. The amount of moisture present in the atmosphere during the Thursday to Friday morning time span will be very impressive. It will be more representative of an air mass we would typically see in the spring or summer. That is why heavy rainfall will be a distinct possibility later in the day Thursday.
This image shows what the Weather Prediction Center is expecting in terms of total rainfall through the entire duration of the event. Most of northern Illinois and southern Wisconsin should see at least 1" of rain, with many areas easily exceeding that total. More rain is likely to fall once you head north and west of Rockford, with slightly less in the way of rainfall as you move south and east towards DeKalb, Rochelle, Sycamore, Genoa, and Paw Paw.