Video speaks for itself, I've dumped a lot of time in figuring out how to finally achieve this in game without using 10meg WAV file. (if anyone ever looked, there is one of those in the game that was abandoned). Cloud textures are mine and this is not using the APH Light Pack. Everything is a WIP.
I plan on phasing out the default rain, but I'm looking for feedback, maybe some suggestions for other samples to call randomly. It's all in the imagination and quite achievable. Not just for thunderstorms, for any weather pattern. I've gone this far so I don't plan to stick to just thunderstorms.
http://www.youtube.com/watch?v=NSEAU5a3jc4

