Open the animation window. For the first frame without moving anything just unlink the content and mask the thumbnail in the snow layer using the layer palette.



Duplicate frame. Move the snow downwards.



Click the tween button from the animation window. Apply settings as shown.




You should have a nice set of frames for the animation done up by now.  See below for an example.




Press Ctrl+Alt+Shift+S to save an optimized animated GIF of your work. Anyway, here is the final result.