Here is a short write up on how to 'snow' on your diorama.

Materials
1. Bicarbonate Soda (cake section from supermarket or provision shop)
2. Glue (Transparent type or PVA white glue, the type that dries clear )
3. Water
4. A small plastic spoon to layer the snow

Step 1

Mix the Soda with water and glue, stir until you get a paste
(Add more soda to have a thick texture so that it is easier to control over the terrain)

Step 2

Layer on the snow onto the surface.

This is what it looks like when it is still wet.

When it is dry
(but while the snow is a bit damp.. press your model into the snow so that it sinks a little into the snow)

Step 3

Once the bottom snow is dry.. it is time to snow.

Coat the diorama with Klear/Future (so that the surface is damp to stick the soda while it is blown onto the surface)

But, I am not sure what other alternative to use other than Klear.
Gloss coat may dry too fast before the snow has fallen.

Anyway, once the surface is damp, pour the soda onto a piece of paper and use an airbrush or use your mouth to blow the soda upwards of the model so that the soda can land downwards onto the diorama (snow falling)

After that.. leave it for a day to dry, and you can flat coat over the whole diorama..

Completed:

Rest of the pics are here.