骤 (zhòu) is a literary adverb meaning 'suddenly' or 'abruptly', often seen in formal writing or news headlines paired with verbs like 变 (change) or 降 (drop): 骤变 (sudden change), 骤降 (sudden drop). It can also combine with 然 to form 骤然 (all of a sudden). Note that the same character has a separate, non-adverbial sense meaning 'trot (of horses)', but that usage belongs to a different entry and is rare.