Here's a very helpful article on photographing lightening. Be sure to read the safety tips as it can be dangerous if you don't know what you're doing.
http://www.digicamhelp.com/how-to/nature/photographing-lightning/If you're shooting in the daylight, you probably shouldn't be using the nighttime setting. Depending on how light, you may not even have to do a long exposure. Try centerweighted metering and experiment with different values of minus exposure compensation to bring out the details of the light colored lightning bolt against a light sky. Take some test shots to see which settings work best.
