You can use net.minecraftforge.client.ICloudRenderHandler to modify cloud rendering and net.minecraftforge.client.IWeatherRenderHandler to modify rain and snow.
You might want to look up examples and usages for more information because I cannot be of any more help here.
I already know how to sync it to the Client from the Server, that's not a problem. But I can't get it to sync from the Client to Server with Entity#hurtMarked, that's the problem. If I try to get the delta movement on Server, the vector looks like this: (0.0, 0.0, 0.0) so saving it and then applying it to the Entity just makes it stop.
Are there any other methods Minecraft uses for syncing to Server?