Global warming intensifies the "hydrological cycle" - the process in which water evaporates into the air, forms clouds, and then rains back down on the Earth.

Higher temperatures cause evaporation to occur more quickly. This can cause very dry conditions on land, even drought. But there's another side to it. The greater amount of water vapor that a warm atmosphere can hold causes wetter clouds to form, so the rain, when it comes, can be unusually heavy - heavy enough to cause flooding. This intensification of the hydrological cycle causes some seasons to be very wet while others are very dry.

We can't say for sure that global warming caused the unusually heavy rain in the Midwest - or any specific weather event. But we can say that the probability of torrential rainfall is increased due to global warming.