I have been to Disney in May for the last three years. In 2001 and 2002, I went the weeks of the 13th and the 12th, respectively. In 2003, I went the week of the 19th. There was much more rain the week I went in 2003. I don't know if it was because it was a week later and getting hotter, more humidity, etc. In 2001 and 2002, I believe there may have been one day out of six that it rained and it wasn't an all day thing either.