Today, NOAA released the official outlook for winter (December-February). Not much to report for Illinois. The northern third of the state has a slightly increased chance of a warmer than normal winter. And northeastern Illinois has a slightly increased chance of a drier than normal winter. However, the increased odds are very weak and we are on the margins for those areas. Historically, the core areas with higher odds on the NOAA maps (for example, the Northwest for temperatures) are more like to be proven correct.
Otherwise, the chances are even-Steven across Illinois for the three categories of above, below, and near-average winter temperatures and precipitation. Maps below.
One of the most important factors for this winter will be the presence and strength of a possible El Niño event. Currently, there is a 70 to 75 percent chance that El Niño will arrive sometime this fall or winter. El Niño occurs over the central and eastern Pacific Ocean and involves changes in the atmosphere and ocean circulation. This, in turn, can affect our winter weather in the US.
Right now the official forecast indicates a weak El Niño, hence the anemic odds over Illinois. If it had been stronger, the odds of a warmer-than-normal winter would be higher for us. A weaker El Niño event could mean that other factors have a chance to play a bigger role in our winter weather. However, things like the Arctic Oscillation and the related Polar Vortex are much harder to predict more than a few days in advance.