godan verb (-su), intransitive verb
begin to rain; begin to snow
Intransitive compound verb used when rain or snow starts falling. Often used with 雨 or 雪 as the subject.
急に雨が降り出した。
It suddenly began to rain.
雪が降り出す前に帰ろう。
Let's go home before it starts to snow.
Compound of 降り (continuative form of 降る 'to fall') + 出す 'to start doing'. The 出す suffix adds the meaning of beginning an action.