>This is part of the April 3-5 Severe Weather Outbreak series.
A cap is a region of negative buoyancy which suppresses the development of storms.
A strong cap is hard to break, thus storms may not develop.
A weak cap is easy to break, thus storms are likely to develop.
Regions with more negativity on the map indicate a stronger cap.
Below is a map of CIN (convective INhibition).
Here, we see a very strong cap over the Plains. That will be working to hold warm air in and make the atmosphere more unstable. Using this map, we can determine the Plains will likely get the brunt of this.
In the Souther Great Lakes, we see a much weaker cap. With that as weak as it is, it can be determined the worst weather will likely not occur there. That may be why the NWS office in that region only has North Illinois down for showers.
We will wait and see.