If you can find no other way - you could try making large wall type static entities I mean large - they can cover numerous tile areas - Make them invisible and without collision.
To these you can attach a decal - you will have to experiment with how the decal is scaled if you can scale it at all - one diffficulty is that the decal texture would distort if the entity it is applied to it is not square so it would have to be, alternativly a single texture (frame) decal that was itself made to the same proportions to any entity applied then resampled to be a square may work.
None of that may work without some experimentation - overiding the way decals work by default in FPSC is not easy - it may be hard work but you can achieve some forced decal effects.
I have not tried out snow in FPSC so dont know if theres an easy way by default. If not its probably best to use the default method even if its via applying may decals as any other experimental method is likely to be just as much work if not more and perhaps not very successful
Theres a snow tutorial around somewhere I believe which may help you - search for it.
