This is the 4967th most frequent Japanese word.
突如
Suddenly; abruptly.
The word '突如' conveys an unexpected and abrupt occurrence of an event, in this case, the start of rain.
突如、雨が降り始めました。
Suddenly, it started to rain.
Here, '突如' is used to describe the unanticipated and immediate action of the subject leaving the room.
彼は突如、部屋を出ていった。
He suddenly left the room.
In this sentence, '突如' indicates the surprising and instantaneous nature of someone's appearance.