Actually, the problem is that the lighting is out in front of the storm line usually. So if you see a storm approaching, the lighting danger is present much sooner than the storm. IIRC, the ground strikes can be as much as 5 to 10 miles out in front of the actual storm line and can come from a even a blue sky. It really easy to get visually focused on the storm line as it's evident in the clouds, but it's out in front of that where the real danger lies.

That's why having one of the apps that shows lightning strikes is important. You can look at that and see where the line of approaching strikes is and know if the problem is heading your way or not.

J.