I live in a fairly dry climate, so I cannot say how the baking soda will react to high humidity, but I think baking soda is pretty inert stuff. As for fixing it, I simply sprinkle it on, blow it around a little to establish realistic drifts, and then spray it with a flat clear. Repeat as necessary to get the amount of snow you like...
I have inserted an image of an early winter (lets say November) snow scene I did - it is the five year old one I mentioned earlier, and I think the baking soda has held up well. Sorry about the picture quality, but I was too lazy to take a better picture and just used a low-rez one I had sitting around. If you need better pictures, I can get you some...
Happy Modelling.
