What weather system? I thought this idea was long time ago discussed and in the end we came to a conclusion it's pretty pointless. I mean different positive / negative stats based on the weather.
my suggestion would be to give the target invulnerability for 1 second. that way you could use it to make a target lose aggro, dodge an enemy spell or dispell effects from him. very versatile. also if its based on the priests heal, its amazing.
Okay, so heal, armor, magic damage + 1 sec inv? Sounds pretty overpowered for me.