I'm not sure what tells the client that rain or snow is falling. Temporarily changing the biome of a NoFreeze lot from a winter biome would prevent snow from falling, and rain would show up instead.
However, I do not know how to selectively prevent this from happening per region. If someone else is able to find out how, they are welcome to post here. Perhaps I should look into how this works and see if it's possible, but as for now, I don't know how this could be done.