Okay, in some biomes, it always rains, while in others, it always snows. My idea is to find a way of setting up the game so that a certain region or selection is forced to snow or rainstorms, independently of biome. A way to change the biome flag of a region without changing any blocks, and thus create a similar effect would also be welcome. I just want to be able to control whether it rains or snows in any given area, and override the default by some means.
It is impossible with a server mod. The client uses the seed to check what the biome is and when it rains/snows it will do that accordingly to the biome. (So only snow in snow biomes and rain in the other.) No way to override this on the server side.