Illinois has an increased chance of being cooler-than-average for August, according to the NWS Climate Prediction Center. The new forecast was released on July 31, 2015.
One of the major drivers in the forecast is that we are currently in an El Niño event. Historically, August temperatures tended to be cooler than average during El Niño events.
This forecast of a cooler August follows on the heels of our cool July. Besides El Niño, in the past the temperature pattern experienced in July has a tendency to carry over into August. That is a hot July tends to carry over to a hot August and a cool July tends to carry over to a cool August.
This is largely the result of soil moisture status. Dry soils in July (i.e., drought) leads to warmer temperatures in both July and August. On the other hand, wet soils in July hold down August temperatures as more solar radiation goes into evaporating and transpiring water back into the atmosphere and less into heating up the atmosphere.
While this concept works great for temperature, there is no similar connection between July and August precipitation in Illinois. The two months seem to behave independently. Frankly that one is harder to explain. In addition, El Niño appears to have little influence on August precipitation.
In any case, while the NWS expects wetter conditions to persist in states west of Illinois, they find no signal in Illinois. Therefore it is left as EC for equal chances of above, below, and near-average precipitation.
While we are just into the 4th day of August, so far temperatures are running 2 degrees above average with very little precipitation around the state. But that is hardly an indicator of the entire month.