na-adjective, noun which may take the genitive case particle 'no'
sudden; abrupt; unexpected; improvised
Describes something happening quickly and without preparation, often with a nuance of being hasty or offhand. Used for both events and ad-hoc actions.
にわかの雨でずぶ濡れになった。
I got soaked by a sudden rain.
彼はにわかの思いつきで旅に出た。
He set off on a trip on a sudden whim.
impromptu skit
Abbreviation of 俄狂言; a short, improvised comic performance. Sometimes written with the ateji 仁輪加.
See also: 俄狂言
祭りでにわかを披露した。
They performed an impromptu skit at the festival.
bandwagon fan; fair-weather fan
Slang abbreviation of にわかファン; refers to someone who suddenly becomes a fan only when something is popular or successful, lacking deep knowledge or loyalty.
See also: にわかファン
優勝した途端ににわかが増えた。
Bandwagon fans increased as soon as they won the championship.
An incident suddenly happened around him.
That story is hard to believe right off the bat.
They say there'll be a sudden shower in the afternoon.
突発 is used for sudden, unexpected outbreaks (like accidents or incidents), while 俄 is a broader adjective for anything hasty or unprepared.
にわか describes someone who suddenly and shallowly latches onto a single interest, while ミーハー describes a person who chronically chases whatever trend or fad is currently popular.
mushroom millionaire; newly rich person
bandwagon fan; fair-weather fan
Ateji spelling sometimes used for the impromptu skit sense; search-only.