Rainstorm/ˈreɪnstɔːrm/ Definition 1 of 1Pro Tip 1/3Intensity Variation The word 'rainstorm' can imply different levels of rain, from moderate to very heavy. The forecast predicts a mild rainstorm this afternoon. Pro Tip 2/3Unexpected Event 'Rainstorm' often signifies an unexpected or sudden event that can disrupt plans. Our picnic was ruined by a sudden rainstorm. Pro Tip 3/3Weather Descriptions Use 'rainstorm' to describe specific weather conditions that include significant rain. The weather report mentioned a rainstorm moving in by evening.