Strong El Niño conditions, with Niño 3.4 anomalies exceeding +2.5°C and forecasts for further intensification through fall 2026, represent the dominant influence on trader positioning for Seoul September rainfall. Historical analogs and multi-model outlooks from agencies like APCC and Korean meteorological sources indicate elevated odds of below-normal precipitation across the western North Pacific and East Asia during this phase, as enhanced convection shifts eastward and subtropical high pressure strengthens. Average September totals hover near 140–170 mm based on 1991–2020 normals, yet interannual variability remains high due to typhoon tracks and monsoon remnants. The tight clustering of market-implied odds around the 150–200 mm range reflects this balance of climatological baseline against the drying signal from the developing El Niño, with upcoming September model runs from NOAA and KMA likely to refine probabilities.