I agree on the snow falling in the foreground. You need that. A great way to get snow in your trees is to take the snow and mud terrain material, change the brown of the mud into a nice dark green, and apply the material to your tree. If you don't have Poser (and I don't, so I do this a lot), see if you can't get some neighborhood kids to pose for you in costume on a cloudy day and insert them as picture planes.

Could be worse, could be raining.