骤 is more literary/classical than 突然 (tūrán). It often appears in formal writing, news reports, or literary contexts. Common compounds: 骤然 (zhòurán, suddenly), 骤降 (zhòu jiàng, sudden drop), 骤变 (zhòu biàn, sudden change).
Left 马 — pictograph of a horse with mane, the indexing radical placing 骤 in the equine family with 驰 (gallop), 骑 (ride), 驾 (drive a carriage). 骤 originally meant 'horses galloping in tight formation,' an image of sudden speed and surging movement — source of the modern 'suddenly, abruptly.'
Right 聚 supplies sound and meaning. Sound: jù → zhòu via old palatal-affricate alternation. Meaning: 聚 means 'to gather, to crowd together' — the horses surge as a pack. The combined image is 'a crowd of galloping horses,' delivering the 'sudden, abrupt' sense through the suddenness of a charging herd.
