After the third wettest July on record in Illinois, the NOAA Climate Prediction Center released their latest August outlook on Saturday. They have the northern two-thirds of Illinois in this region with an increased chance of above-normal rainfall in August (first map, green shading). This is part of a larger area with increased chance of above-normal rainfall that extends into Iowa, northern Missouri, southern Minnesota, Nebraska, and South Dakota. While not part of the official forecast, many of the dynamic forecast models are showing that above-normal rainfall is more likely in southern Illinois as well.
The Climate Prediction Center also shows the southern third of Illinois with an increased chance of above-normal temperatures in August. See second map.
For Illinois, it is rare to be both wetter and warmer than normal during summer. The rainy conditions tends to hold down daytime temperatures due to increased cloudiness. In addition, abundant soil moisture means more solar energy can go towards both evaporation from the land and water, and transpiration from crops, with less energy available for heating up the atmosphere. We saw the effects of rainy weather in July, with only a few really hot days but lots of humidity.