I know the phrase "raining cats and dogs" but I don't know that I've ever heard anyone actually say it in conversation outside of my elementary school teachers! LOL Usually someone just exclaims "Holy $hit, look at the rain!" or "Damn, that's comin' down hard isn't it?"

As for the 'underwear in a snit' variation for my area - it's "panties in a bunch", as in " Geez, don't get your panties in a bunch!"