Traders see near-even odds across 75–250 mm+ bins for Seoul’s September total precipitation because of high uncertainty in the East Asian monsoon transition and Western North Pacific typhoon tracks. Historical averages range 150–180 mm, but recent decades show rising early-autumn rainfall linked to a delayed North Pacific High retreat and more frequent tropical cyclone influences. Late-August 2026 stationary fronts and warm Yellow Sea waters produced extreme localized totals exceeding 500 mm elsewhere in Korea, underscoring volatility that could extend into September. Official outlooks favor near- or below-average conditions, yet model spreads on steering patterns and any late-season storms keep multiple bins competitive until clearer signals emerge from updated forecasts.