Mmm don't know! I've seen footage from US supercells with strikes way, way out under the anvil and grounding and these anvils were spread back from the core many miles from it. It is recorded that a strike was grounded 42miles away from a storm in the US - but we're all correct when we say that it's an unpredicable force. One of those things. I have seen a CG exit from the very tip of an anvil to the ground around the tower and this tower was huge - i don't know what the height of the storm was but i'm talking about strikes emitting from a storm and travelling outward 20km and not 'vertical' to the storm itself.
Even the event here on March 1 2007 with that deepening tropical low, no strike travelled that far and we had over 2000 in a space of two hours and that included smooth, branched CGs and that system had an astounding number of large/severe storms imbedded in it.
I have not seen CGs that strong emit from any dissipating storm here. I photographed a series of storms across the harbour the other night and they were at least 30km away. All of the CGs were active within a westerly positioned rain shaft/updraught area, whilst the anvil was spread out to the east - whilst the eastern side of the storm appeared to be dying out and only produced crawlers, the updraft was still constant enough to keep active CGs within the core for over an hour and a half....
I have no doubts that it happens, i just have not seen anything on it as yet except for footage and how do you gauge distance unless your'e the one videoing it!
Mike