I was driving north on rt.24 when I noticed what looked light lightning in a cloud formation. I enjoy watching thunderstorm so I am quite aware of how they behave. Initially the flash behind this cloud looked like lightning, but that was not the case. It flashed on very quickly, but it only lit up one cloud instead of the entire cloud bank. It also grew ateady in intensity and then faded slow to nothing. The lengthof time and the fact that the light did not flicker or fluctuate in intensity ruled out lightning in my mind. I have NEVER seen lightning do this in my life. I did not see an object associated with this, but I have not idea what could have caused this.