According to the National Weather Service Hydrometeorological Design Studies Center , the amount of rainfall in the hardest - hit locations had a less than 0.1 percent chance of happening .
Was a ( less than ) 1 - in - 1,000 - year event .
According to the National Weather Service Hydrometeorological Design Studies Center , the amount of rainfall in the hardest - hit locations had a less than 0.1 percent chance of happening or was a ( less than ) 1 - in - 1,000 - year event .