El Niño conditions strengthening across the equatorial Pacific represent the dominant large-scale driver, with multi-model ensembles projecting continued intensification through September 2026 and associated shifts in the western North Pacific high that can steer tropical cyclones or alter moisture transport toward the Korean Peninsula. Seoul’s climatological September total averages 141.5 mm (1991–2020), concentrated in the early month before the East Asian monsoon fully retreats, with high interannual variability from typhoon landfalls. Recent seasonal outlooks indicate mixed signals—some models favor below-normal totals while others highlight enhanced East Asian rainfall probabilities—leaving genuine uncertainty around intensification windows and steering patterns. Trader consensus clustering near 175–200 mm reflects expectations of near- or modestly above-average accumulation from residual monsoon or storm activity, tempered by the inherent spread in subseasonal forecasts.