The Washington Post, in a detailed analysis of this flood, has this to say about the role climate change played:
Climate change did not “cause” this thunderstorm complex.Emerging patterns are undeniable, revealing climate-change deniers for the antediluvian fools they truly are.
However, climate change has probably altered the larger environment in which these small thunderstorms are embedded. Notably, the water vapor content of the atmosphere, as a whole, has increased and scientific studies have shown a statistically meaningful uptick in the frequency of extreme rain events over the eastern United States. Statistically, over the long term, these types of extreme floods are probably becoming more common, in areas that are normally rainy as a result of global warming.