There was some talk about this a while back, but it's just a lot of work

. One thing that we may be able to implement pretty easily is a sort of client mask where we can put rain falling down as a large mask over the gameplay area in the client. But as far as actually having a weather system ingame (where you can see the sun in the water, for instance)... that's probably not going to happen (unless someone takes it on).
Cheers,
Unislash