Maybe its a just a mormon thing.
But everyone mormon I've ever known took it this way:
After the Flood, god put a rainbow in the heavens to show hes not going to do that again, its cool.
Then they took it a step further with, when you don't see rainbows anymore... gods gonna wreck things again.
Now there are plenty of teachings you hear about in church or in seminary school as a teenager, that are super common and people say, but aren't really nailed down, kind of like the whole "God lives on a planet called Kolob" or "when you die if you are good and get to the best kingdom you get to be a god and have your own planet" but they arent very clearly written down in many places, partly because I think even the church realizes they are patently ridiculous and wanna avoid that bad PR.
But here:
Talking about the 2nd coming