Hi Rob,
poured plaster is the best way to do deep snow. A dusting of some glittering model railroad snow powder , fixed with diluted white glue, puts the icing on the cake
Depending how experimental you are, you can also spray blue shades on the plaster before the "icing". This gives the snow a much colder fell than the plain white of the plaster.
Below is the link to the snow base of my current project:
http://news.webshots.com/album/574017168PvUenK?start=12
cu
Stefan