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sudden; abrupt; unexpected; immediate; hasty; improvised; impromptu; offhand
impromptu skit
bandwagon fan; fair-weather fan
にわかに空が暗くなり雨が降り出した。
The sky suddenly darkened and it started to rain.
にわか雨に降られて傘がなくて困った。
I got caught in a sudden shower without an umbrella.
彼のにわかな態度の変化に驚いた。
I was surprised by his sudden change in attitude.