Rain and snow should be easy to add to multiplayer since they don't need to be synced with the other players. I mean, the chance of a snowflake falling in front of you and blinding you is a pretty low chance.
As for lightning, why bother actually syncing it? Just pre-randomize about 20 to 50 lightning strikes at certain time intervals over a 5 minute or more period, transmit those lightning bolt times to any player entering the game or starting the game along with the host, and just loop those for your lightning strikes. You could sync them locally to the in-game timer and wouldn't have to worry about transmitting every lightning strike over the network since every player's game already knows when each lightning strike will occur, even though the player won't. You could also randomly select the time period ahead of time so that it would be nearly impossible to predict every lightning strike.
Mind you, why go to all that trouble even? I would think this would just be a gravy feature to add if 1.1 gets done ahead of schedule.
--- Gemini