noun
sudden shower; sudden downpour
A brief, often intense rain that starts and stops abruptly. More literary than にわか雨, which is the everyday word.
夏の午後、驟雨が通り過ぎた。
A sudden shower passed through on a summer afternoon.
驟雨に降られて、ずぶ濡れになった。
I got caught in a sudden downpour and was soaked.
From Middle Chinese 驟雨 (dzrjuwH hjuX). The first character 驟 means 'sudden, rapid' and the second 雨 means 'rain'. The reading しゅうう is the on'yomi.