**Trader sentiment for Seoul's September precipitation heavily favors the sub-75 mm outcome (47.5% implied probability) due to the dry start to the month and model consensus for limited additional rainfall.** Historical normals place September totals near 140 mm (1991–2020 KMA data), driven by the retreating East Asian monsoon and occasional typhoon influences, yet early September 2026 observations show mostly clear, sunny conditions with negligible accumulation. Current KMA and ensemble forecasts indicate continued stable high-pressure dominance and minimal convective activity through mid-month, reducing the chance of significant late-season contributions. While westward-shifted typhoon tracks and delayed Western North Pacific subtropical high retreat have boosted autumn rainfall in recent decades, no active systems threaten the peninsula in the near term. Key upcoming catalysts include updated KMA extended-range outlooks and any tropical cyclone developments that could shift probabilities toward the 100–125 mm bin or higher.