"Bomb" is a description that is used fairly often in colloquial meteospeak. It can get applied to just about anything that could be loosely described as, well, a "bomb," with a "rain bomb" being one application of the word.

More formally, when discussing very heavy rain events, mets may describe them as "QPF Bombs" - or Quantitative Precipitation Forecast *Bombs*, when the models forecast and/or verify such events.