I find it helpful to look at a weather web site that has an hour-by-hour forecast. That always helps me plan. That said, I always bring at least one hoodie and pair of long pants, regardless of the forecast. If the weather ends up taking a swing toward cooler temps. At least you don't have to spend big bucks on WDW apparel if you need something warm. Better to bring it, and not need it, then wish you'd brought it!
(I once went on a WDW vacation that spanned almost two weeks. It was October, and the first part of the forecast, for he first week, showed highs in the mid-90s and high humidity. I packed almost all shorts, sleeveless tops, and sandals. Well, the end of my second week, we had an unseasonable cold snap, and what I wouldn't have given for some warmer clothes! The temps dropped 30+ degrees almost overnight (and was quite windy!), and I was faced with having to buy some pretty expensive clothes to feel comfortable. Now on trips, I bring a little bit of everything!)