にわかに・俄かに・俄にniwakani[1]atamadaka (head-high)

1. adverbsuddenly; abruptly; unexpectedly
   にわかに雨あめが降ふってきた。↗

   Suddenly, the rain fell.

• Synonym突然とつぜんNeutralThe most common everyday choice, at home in speech and writing alike, and usable as a noun or adjectival noun too. 「俄かに」 is more written and mostly seen in prose.
• Synonym突如とつじょWrittenWritten register, stronger than 「突然」, as in 「突如現れた」. Roughly interchangeable with 「俄かに」 but lays more weight on the total absence of warning.
• SynonymたちまちNeutralFocuses on a state being reached in the blink of an eye, as in 「たちまち売り切れた」. 「俄かに」 stresses that something happens out of the blue.
• Synonym不意にふいにNeutralHighlights catching someone off guard with no chance to prepare, as in 「不意に声をかけられる」. 「俄かに」 is more written and speaks of a sudden turn of events.
• Synonym急にきゅうにNeutralCommon in both speech and writing, as in 「急に雨が降る」. It can replace 「俄かに」, which is the more literary of the two.
• SynonymさっとNeutralCovers swift, crisp movements or the sudden rise of wind and rain, as in 「さっと隠す」. 「俄かに」 stresses the unexpected, not speed.
• SynonymパッとNeutralColloquial; captures something happening in a flash, as in 「パッと明るくなる」, and can also mean eye-catching or showy — senses 「俄かに」 lacks.
• Synonym急激きゅうげきNeutralAn adjectival noun for changes that are both rapid and intense, as in 「急激な変化」. 「俄かに」 stresses the unexpected, not intensity.
• SynonymいきなりCasualA common colloquial choice for doing something with no lead-up, right from the start, as in 「いきなり殴る」. 「俄かに」 leans written.
• Synonym俄然がぜんNeutralDescribes a sudden change in a situation or attitude, as in 「俄然やる気が出る」; colloquially it means 'by far' or 'absolutely'. 「俄かに」 leans written and stresses the unexpected.
• SynonymふいとCasualColloquial and uncommon; describes doing something abruptly on a whim, as in 「ふいと立ち去る」. 「俄かに」 is more often used of situations that change suddenly.
• Synonym突としてとつとしてWrittenWritten and old-fashioned, rarely met in everyday language; roughly equivalent to 「突然」. 「俄かに」 also leans written but is the more common of the two.
• SynonymふっとNeutralSuggests a light, fleeting moment, as in 「ふっと消える」 or 「ふっと思い出す」. 「俄かに」 carries none of that lightness.
• SynonymついとNeutralDescribes a quick, single movement, as in 「ついと立ち上がる」, though it is uncommon. 「俄かに」 is about things happening unexpectedly.
• SynonymひょいとNeutralSuggests a light, casual movement or something done by chance, as in 「ひょいと現れる」. 「俄かに」 has no sense of lightness or chance.
• SynonymばっとCasualA colloquial mimetic word for bursting into action or scattering at once, as in 「ばっと飛び出す」. 「俄かに」 is a written-style adverb.
• Related急きゅうNeutralAn adjectival noun meaning sudden or urgent, as in 「急な用事」; as an adverb it becomes 「急に」. 「俄かに」 exists only as an adverb.
• RelatedふとNeutralCovers thoughts or actions arising without conscious intent, as in 「ふと思う」. 「俄かに」 describes situations changing suddenly, with no sense of the unintentional.
• RelatedやにわにWrittenWritten and dated; stresses acting on the spot, at once or without warning, as in 「矢庭に走り出す」. 「俄かに」 stresses that events happen unexpectedly.
• Related忽然こつぜんWrittenWritten; focuses on appearing or vanishing in an instant, as in 「忽然と姿を消す」. 「俄かに」 is not limited to appearing or disappearing.
• RelatedとみにWrittenWritten; stresses a change becoming striking or ever more pronounced, as in 「頓に増えた」. 「俄かに」 stresses sudden occurrence.
• RelatedつとWrittenWritten and dated; means quickly, and can also mean staying motionless; rare today. In everyday speech say 「さっと」 or 「急に」.
• Antonym徐々にじょじょにNeutralThe antonym: change that comes gradually, little by little, as in 「徐々に慣れる」. It sits at the opposite pole from the suddenness of 「俄かに」.

For everyday talk of the sudden, reach for 「急に」, 「突然」 or 「いきなり」. 「俄かに」 leans written and often appears in set phrases like 「俄かには信じがたい」, hard to believe at once. Among the near-synonyms, 「突如」 and 「忽然」 are likewise written, while 「たちまち」, 「さっと」 and 「パッと」 stress speed rather than surprise, and 「ふと」 the unintentional. The antonym 「徐々に」 marks gradual change.

Usage comparisons are AI-assisted and cross-checked by multiple models; for study reference only

1. 予想よそうしないうちに、急きゅうに起おこること。