I've seen something about this before, but I cannot remember where...
Usually, in order to get snow on a building, I simply make up a duplicate of a texture, paint the roof parts white, and rename the texture with a _WI suffix. This makes a nice flat snow effect on the roof. But I want to simulate six or so inches of snow on the top of a building. I know that you can make objects appear only at night by using the fx_night heirarchy. Is there one for winter or snow?