its too ez lol
all u do is go into photoshop
open up ur animations window
make some fake snow with a brush..u can just put small ass speckles everywhere
u wanna make like 3 different ones so it looks different(make sure its on 3 different layers)
now on frame one hide 2 of the snow things and extend that fram out how many ever frames u want it
then on the next fram hide the first snow u made and unhdie the second one and extend that out how many frames u need
then on the last snow pic hide the first 2 and unhide u last one and ofc extend that out to how many frams u need it..and then press the play button
it looks like snow falling