Thanks for asking--we used 3 different snow plates from Digital Juice's Compositor's Toolkit... usually two layers at a time, screen transfer mode, one bigger and out of focus and one further away and left sharp. Most of it was composited quick 'n dirty either in Final Cut Pro or Motion, but the finale "pull away from the door" shots were done in After Effects using single-point motion tracking with multiple layers (up to 4) of snow, combined with high-feather keyframe-animated mattes of the people closest to the camera in order to give it a sense of realistic depth.