I am a bit skeptical if this will work.
The image sequence here File:Lightnings sequence 1.jpg - Wikipedia shows 8 frames taken over 0.32 seconds. That works out at 40mS per frame. I would have thought you only had two frames here that would trigger your unit, that's 80mS, very close to your delay.
It depends on how soon you will be able to detect the bolt as it grows. This means that the trigger level is a vital aspect.
How about connecting a pot to one of the analogue pins and using that to set the threshold. In that way the next time you have a storm you can easily adjust the level without having to download to the Arduino all the time.
Anyway prove me wrong and post a picture after you find a storm. 