A 256-year daily rainfall record reveals multiscale rainy-season variability and emerging hydroclimatic hazard over northern East Asia.

Monsoon rainy seasons shape water supply and flood hazard across northern East Asia, but their long-term variability remains poorly understood because few daily rainfall records extend beyond the instrumental era. Here, we reconstruct rainy-season onset, retreat, and duration from a 256-year daily precipitation record for Seoul (1770-2025), providing an unusually long view of seasonal monsoon behavior. The reconstruction reveals structured quasi-decadal to multidecadal variability in rainy-season duration, indicating that recent rainy-season lengthening over northern East Asia is consistent with the latest positive excursion of a recurrent multidecadal modulation rather than a purely isolated modern anomaly. Coupled with warming-associated intensification of extreme rainfall, this prolonged-duration phase has elevated the compound-hazard probability, with recent probability peaks exceeding the long-term mean of ∼25% in Seoul. Exploratory near-term outlooks suggest that rainy-season duration in northern East Asia is likely to remain prolonged toward mid-century, sustaining heightened compound hydroclimatic hazard when seasonally extended overlaps with intensified event-scale rainfall.