cropping more off the top of the pic may help improve this one, and I'd like to see the colour version of this shot too. This photo seems to give the impression that a lightning will appear anytime at all :)
COMPOSITION... Sometimes when trying to shoot a great atmospheric shot, attention is sometimes lost to the foreground... which can be just as important to a photo if it is included of course... I just find this shot could have used a little more of it.... putting in just a little more of the city line by tilting the camera down (filling one third of the bottom ideally)... or even better, finding some large object in the landscape (interesting rock, sculpture) to shoot it against. You definitely got a great foreboding sky though and it aptly fits the challenge.
TECHNIQUE... A levels adjustment should be employed to give proper contrast. A histogram shows the highlights are not close to pure white. Other than that, image quality suffers slightly... the file size is 52 kb. When saving the file as a jpg, use the highest quality (lowest compression) possible such that the file size does not exceed 150kb.
I think it has just a little too much on the top. The top third or forth of the frame isn't nearly as exciting as the bottom part. Without it it would have been an 8 from me. But as is, a 7.