Just use the Trigger Zone event. Works in multiplayer. Set the TriggerID to whatever TriggerID, and "Switch" and "Onoff" to 0. If your JCS.ini doesn't have Trigger Zone (It would be in Gameplay: Area or Gameplay: Triggers and Scenery) get one that does.

As for the rain/snow tiles, put them in a seperate layer. Tile them. The auto-Y speed is how fast it will go up, so set it to negative for falling. Left is positive auto-X speed, and right is negative.