I too intend to model snow on my US layout, and have done a fair bit of research on the topic. I've been advised that marble dust is not the sort of stuff you want to be inhaling.
I intend to go with Woodland Scenics snow which apparently will look a bit grubby before too long and will need refreshing. If it gets dirty I'll vacuum it up. Unfortunately that'll get a bit costly, but I do like the idea of having fresh snowfall in different places from time to time. I don't want a heavy snowfall, so I doubt I'll be using putty or plaster, although experiments suggest that dry plaster powder looks pretty good when scattered around.
There's bare plaster in this photo, mixed and used as landforms. It's not intended to be an example of good snow, but I think it does show potential. Chiselled or sanded, it can look quite convincing, in my opinion. Certainly when I've sprinkled a little plaster powder over grasses etc it has looked pretty good. I'll have to experiment some more.
Mike
I intend to go with Woodland Scenics snow which apparently will look a bit grubby before too long and will need refreshing. If it gets dirty I'll vacuum it up. Unfortunately that'll get a bit costly, but I do like the idea of having fresh snowfall in different places from time to time. I don't want a heavy snowfall, so I doubt I'll be using putty or plaster, although experiments suggest that dry plaster powder looks pretty good when scattered around.
There's bare plaster in this photo, mixed and used as landforms. It's not intended to be an example of good snow, but I think it does show potential. Chiselled or sanded, it can look quite convincing, in my opinion. Certainly when I've sprinkled a little plaster powder over grasses etc it has looked pretty good. I'll have to experiment some more.

Mike