Heat lightning pictures?

Okay so i took pictures of the heat lightning tonight with my nikon d3100, bulb, aperature at 7.1 and ISO at 100. There are a lot of clouds and the best picture i could get on my deck is this, most of it is behind the clouds and it's still really dark. I took advice on the settings from people on here and google but this is the best i could do.

According to the exif data on the picture the aperture was at f/4.5 and the exposure time was only 2.2 seconds. The exposure is too short

Do at least a 30 second exposure or longer. Here is one I did the other day with a 74 second exposure

Interesting image, kind of subtle effect but has a nice tone. Keep up the good work, must be a fun storm if it does not start any fires.