The Washington Post wrote earlier this week about the extremes in weather that have occurred around the globe in May. Scientists attribute the extremes at least in part to an unusually stationary jet stream which has locked in place, bringing torrential rain to Texas and Oklahoma and leaving other areas dry, including much of the Southeast. They link this to the current El Nino, which developed at an odd time compared to its usual behavior. You can read the article here.
The floods in the southern Plains may also be attributed in part to more humidity in the air from rising temperatures in recent years. The flooding in Texas was so big that it ended their long-term drought; you can read an article about this at http://www.wired.com/2015/05/texas-floods-big-ended-states-drought/.