I've discussed Lightning
(well, sorta), now let's talk about Thunder. (Yes, there's a reason I capitalized them.)
Thunder is much slower than Lightning. Meaning, if you see Lightning, Thunder won't be too far behind, but he'll always be behind. He always brings up the rear, never leading. Lightning is always the leader and Thunder the follower.
The closer you are to Lightning, the greater the impact Thunder has. Thunder gains his power from Lightning, and if you are too far from Lightning, Thunder's impact will be almost zero. However, Thunder's impact is not to spread to the direct point of Lightning's strike, as that is meant to be Lightning's impact.
Perhaps you are experiencing Thunder right now. However, if you can sense that his impacts are affecting the vicinity, it is advised that you do not head outside because that means Lightning and his deadly impact are near.
And there you have it. Thunder, the booming, yet slower brother to Lightning. Together, they have a very powerful impact, but separate, Thunder's impact is far lesser.