A fairly active pattern set up has kept storm potential in the forecast for the past couple of days. Once again there is the threat for storms today. There is more of an isolated threat for storms this afternoon, as a potent mid-level wave moves overhead. This typically brings scattered coverage of storms, but this afternoon there is somewhat of a 'cap' over the atmosphere. Think of it like a bottle cap lid, even though there is instability building beneath it, storms have a hard time trying to break the cap.
The storm threat goes up this evening, as a stalled boundary south of the area lifts north, this would help storms to get going as the cap weakens. The air mass over northern Illinois is becoming more and more unstable today, and with the lift provided from the front, it should be enough to get things going across the area. The Storm Prediction Center has expended the slight risk to include northern Illinois and southern Wisconsin. Storms look to develop late today or even this evening, but would still bring the potential for very heavy rain and even large hail.
Make sure to check back for updates to the storm threat this evening!