It's all chance. It has to be raining. Lightning randomly strike a block inside a quadrant that's raining
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I've seen some people enter it in as /Smite <player> through a command block, although I'm not as sure how to do it in Singleplayer mode.
most possibly lightning but the rain will extinguish the flames. your minecraft is possibly giltched.
On a bukkit server, if you have permissions, you can do /thunder, /lightning, /smite or commands like /weather set storm to get the thunder/lightning effect. It all depends on the commands you have on a server. In singleplayer you would have to change the weather by doing "/weather thunder"
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