I started checking long range weather forecasts on the Accuweather site last month. Their "extended" forecasts covers 45 days. At that point, I didn't expect accuracy--only a good estimation of rainy days versus sunny ones--along with getting a general feel for temperature ranges.
This last week I'm getting more realistic forecasts, especially for the first few days after arriving. So far, we are sunny for landing in Rome and sunny for Tuscany, the first leg of our trip. I've also checked the water temperature on the Amalfi Coast and on the Pugliese coast... both are pretty warm for this time of year--around 76 degrees Fahrenheit. Great for taking a dip or wading in a tide pool. Certainly good enough for tooling around in a boat for a while. Oh, and the hot air balloon trip we planned for Tuscany looks good so far... sunny. Can't wait.
UPDATE 5/22/17: In reality, Italy was very hot when we visited. Instead of daytime temperatures in the mid-seventies, we had many days in the upper eighties and some well over 90F degrees. Locals were telling us that the temperatures were "normal" for this time of year. We learned that since Italy doesn't have extreme winters as we do in the northeastern U.S., their autumn is also much warmer than ours.
A mistake we made was bringing a jacket for each of us which we never used. Packing a shell windbreaker and planning on layering would have saved some space in our luggage. One thing we did use and enjoy when we traveled was our Thermos brand bottles that held drinks ice cold for many hours. Carrying a bandana was also helpful (wearing around my neck, wetting in a fountain to wipe my face, etc.). Wet Ones single packs are also great for wiping away sweat and cooling down the neck of an 11 year-old grumpy kid while walking around Pompeii. And I'm a fan of wearing caps and walking on the shady side of streets when possible. Stay cool, mio amico!