The Storm Prediction Center has issued a Mesoscale Discussion for parts of Missouri and Illinois.
Basically, a Mesoscale Discussion just points out an area of specific interest by their meteorologists. Typically, you can get a feel for whether or not they are going to issue a Severe Thunderstorm or Tornado Watch from the discussion.
In the discussion they talk about the atmosphere getting a bit more unstable over parts of southeast Missouri. At this time they are watching for clusters of thunderstorms to develop. If they do, then they could issue a watch.