Compositing Lightning

Hey, everybody. I love this place, it's great to read and see what others are doing. I'm sure someone else has done this, so I'm needing some help. Whenever I get my GH2 (been waiting for over a month) I am planning to shoot the opening scene to my screenplay Dead Generations http://www.generationsthemovie.com

It involves rain and lightning. I have already made a composite of lighting I shot over the years and then just recently shot some rain clouds pre-storm to give me a more scary look. I'd like to put this composite behind the action of this scene.

Shooting at night and dropping off to black I was thinking I could use alpha channels, but can't seem to get it to work. I've tried some composite mode settings, but not get exactly what I want. I thought alpha channels would allow me to use black like chroma key and the black would be replaced by my cloud composite, am I wrong about this and not understanding alpha channels?

A little about this composite. The clouds were shot last week, some of the lightning was shot 4 years ago in Charlotte, NC, some was shot coming back from L.A. while driving through a cool storm in Texas, I pulled off the interstate and shot through my sunroof!
http://www.justasparkfilms.com/Lightning.mov

Looking forward to the comments.

Can anyone suggest a way to do this?
 
Yes, I think you're confusing the term "alpha"... You would want a Luma Key to key out black, but beware of including shadows on your subject in the key.
 
Agreed. You'll probably want a combo of luma matte/keys and masks. You might also want to consider shooting day for night.
 
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