Traveling north on Greenville ( TX-HY 5 ) in Allen Tx. Noticed a single, smallish cloud in the sky(from my best judgment 25-50 yds. wide x 20-30'tall). approx 10 deg. elev. 020 mag hdg. and with no other clouds visible. That seemed a little odd, but i did not give it much thought. When I was approx. 500 yds. from turning onto a side street, I noticed the lone cloud brighten up then a very pronounced flash. It was like a lightning dis-charge that one normally sees, but there was absolutely no other clouds and it did not emit a "bolt" of lightning. Nor did the cloud seem to be a large enough to generate such a bright charge like what i saw. In addition, i have never seen any cloud in solitude flash in the sky. As an aviation "junkie" I tend to look at the sky often and I can usually tell what is what in my area. As well, I am familiar with the (STAR's)and (SID's) that the commercial/civilian aircraft use in this region for the Love Field and DFW airports. I immediately knew it was not aircraft/helicopters so this leaves few options. I only saw one flash and it happened quickly, but i did keep sight of the cloud during my turn and onto the side street until i lost view due to trees. The cloud itself did not seem to have any unusual features other than the left and right sides were symmetrical. Three "humps " upward with the center hump being slightly higher than the two outer "humps". The bottom was typical flat.