There exists an amazing program to layer and loop nature ambience from user imports, SoundSense
It comes with several thunderstorms licensed for it, however, sheesh,I brought this one into the app to play it, and it loops REAL nice. It's now part of my winter Environment collections to get that fix for rain in winter.
What's nice about this app is it can play 100's of sounds at the same time, real messed.
Check this app out, it can really make looping the weather loops a snap: