Here is an old video for CoE. It has the sounds of thunder and rain, as well as you can see the rain. There are also other videos of falling snow. http://media.chroniclesofelyria.com/videos/bbea4fb867bb447687ef936b5df42fbb.mp4
I don't know of media that shows leaves in mid-fall, but this was said:
26 JUN 18 D-CCR
But to be clear, it's not like they don't work wood, either: The forest gives the Kypiq lumber. They would never fell a tree for its lumber, but a tree already felled has no spirit to upset.
So there are going to be falling trees. And I imagine there are going to be falling leaves as you can see individual leaves on the ground in this pic: http://media.chroniclesofelyria.com/screenshots/539763734a534e1ebb90534b24fb7e28.jpg