I always use weather.com to get a 10 day forecast for the cities I am visiting as well as the historical weather for the same period last year.
What I like even better is a webcam so I can actually see the conditions on the ground and more importantly for me, what people are wearing.
https://www.skylinewebcams.com/en/webcam/italia/veneto/venezia/piazza-san-marco.html
Thought I'd share as I feel this is a game changer for me!