Summary: The NWS Climate Prediction Center has issued their forecasts for September, September-November (Fall), and December-February (Winter). Illinois has an increased chance of being warmer than normal this fall, and wetter than normal this winter.
According to the Climate Prediction Center, the main factors in the forecast are the recent warming trends and the expected La Niña. While the conditions in the Pacific are in the neutral stage between El Niño and La Niña, there is a 55-60 percent chance of a weak La Niña during fall and winter.
The September forecast (top row) has Illinois and the Midwest with equal chances of being above, below, and near-normal on both temperature and precipitation. I call this a neutral forecast since there are no indications that we will be significantly cooler, warmer, wetter, or drier.
The September-November forecast (bottom row) has Illinois and the US with an increased chance of being warmer than normal. They are neutral on the precipitation forecast.
The December-February forecast for Illinois is neutral for temperature. It looks like we are sandwiched between warmer than normal conditions in the south and colder than normal conditions in the north. The precipitation forecast has Illinois and the Great Lakes region with an increased chance of being above-normal on precipitation. Click on images to enlarge. Click to enlarge.
The winter forecast reflects the typical La Niña winter pattern for the US with the north being cooler, the south warmer, and the jet stream parked right over Illinois. In the past, that has given us wetter conditions and more snow in the Great Lakes region.
The spring forecast shows the southern half of Illinois in part of a large region with an increased chance of above-normal temperatures. The northern half of Illinois has equal chances of being above, below, or near-normal on temperatures. Meanwhile, the higher chances of above-normal precipitation retracts out of Illinois, leaving us in neutral territory.