That would end up looking like the Christmas Village displays, right?
Isn't that the idea?

Get an old sheet, and cut it into pieces fitting onto your gaming surface. So fields should be covered completely, but roads, streets and other frequently used features should be left uncovered.
Thin sheets of styrofoam might be useful to cover larger areas as well.
You could maybe use a light dusting of spray-on snow from directly above. Depending on how well your buildings are varnished/protected, you could then wipe/rinse it off after the game.
Try getting your hands on bird sand (you know; the stuff used on the bottom of bird cages). Most of it is pretty white, or even completely white, and since it's sand, won't react with your terrain like salt might. It is also easy to shake off and maybe re-use.
And getting some cheap pine/fir trees at the railroad shop and spraying them with white from directly above would give you some wintery trees for not too much money too.
By the way; for the same reason I wouldn't use salt, I would also not use flour or baking soda; they might react with your terrain, potentially ruining it. Stick with the inert stuff.