The OST from Undertale called It's Raining Somewhere Else is referring to Deltarune, and will play at some point in the game.

The name itself is very strange, accompanied by the fact that it sounds like it's raining, meanwhile, when the song actually plays, we are literally in the hottest place in the underground: Hotland. It also plays when Sans talks about being happy with what's given to you. This, alongside his Lost Soul dialogue in the Asriel fight and the "Don't Forget" photo in Sans's lab, really makes me think he is actually from Deltarune and the whole "Sans Undertale is from Deltarune" theory feels very solid to me too.

In conclusion: I think that a remix of It's Raining Somewhere Else is going to play when Kris and Susie are eating and talking at QC's Diner while it's raining outside, and it'll probably be named "It's Raining Outside", or "It's Raining Right Here" or even just "It's Raining". Maybe Kris and Susie are going to have a heart to heart talk, just like the one Sans and Frisk had in Undertale, and it'd be the perfect place for it to play, and if it does, i think i might legitimately cry. This song means a lot to me and is probably my favorite from the entire Undertale OST, so i would absolutely love for it to return like this. Also, it would pretty much confirm the "Sans Undertale is from Deltarune" theory, which i personally really like.