In the game, if you fought inside the Scarlet Devil Mansion, the weather wouldn’t show up at all. And early in the game, there was no translation, so if you couldn’t read the moonrunes, you basically had a disadvantage unless the weather was something totally obvious like typhoon (or, you know, you actually bothered to memorize what popped up on screen). So at the time imagining what would happen if the weather was allowed to roam freely in the mansion was a great idea to play with.