Eastern Montana is not known for its abundance of water. In fact the area has been in a drought for the past 10+ years, and has been struggling. However, so far this year they have received more rain already than they would expect in the entire year, and there is another storm coming to give them a couple of more inches. This has led to a lot of flooding. The Yellowstone is at near flooding all along its course. The problem with this is that the mountain snows have yet to melt. The water causing the melting at the moment is from rain; the snows have yet to start to come down. It sounds like all this could only be adding more water to the Missouri/Mississippi river system.
(BTW I am listening to it raining quit hard as I write this.)