Rumor has it that weather has nothing to do with his spawn timer.
Rumor has it that he's suppose to have a 12-15 hour timer (I've personally camped him longer with no luck)
Between me and a few others, we've successfully camped his spawning location at Cattail Lake for 18-22 hours in solid blocks, and obviously we have no data on whether he's spawned during those couple hours of 'black' time. Please don't troll the thread, if you've legitimately seen him, just a simple server-time and date will suffice, and Congrats on your Charm. Ancient Pandaren Fishing Charm