This may be a case of confirmation bias, but I’ve noticed lately that when a storm rolls into the area, and the skies get dark, it usually doesn’t start to rain until I see lightening and hear thunder… then the skies open up. I live in the Rockie Mountains if that matters, and we get quite a few thunderstorms in the Spring and Summer months.
So is there some relationship between thunder, lightening and the start of rain, or is it just a coincidence?
Could easily be confirmation bias. On the other hand, you can see lightning and hear thunder that are quite a long way away, depending on what kind of valley you’re in or how much land the mountain you’re on overlooks, but you have to be right under a rain cloud to get rained on.
Ha, I was about to make this topic as well. It’s a common cliche in tv/movies. Theres always a big rip of thunder followed by a sudden downpour of a rain.
Well, there’s definitely a non-coincidental relationship between lightning and thunder. 
I’ve noticed it too, but given that rain has to fall quite a while before it hits the ground, it seems likely that if there is reliable causal chain to the timing, it may be:
[ul]
[li]heavy rain falls to the point where it’s nearing the ground[/li][li]the greater moisture content in the air trips the lightning[/li][li]You notice the lightning, hear the thunder, then a few seconds later, the first of the rain finishes its downward journey.[/li][/ul]
If a thunderstorm is moving in, you will see the lighting and hear the thunder before it rains. That’s because they both can be seen and heard from some distance away, while the raincloud has to be over you to drop rain on you.