**The near-certain market pricing on 21°C as Seoul/Incheon’s October 1 high temperature reflects official forecasts and real-time observations of a maximum near that threshold under clear skies and moderate northwesterly flow.** A pressure trough and subsequent cold-air advection from the north dropped daytime highs roughly 5°C from late-September levels, producing afternoon peaks of 20–22°C across multiple reporting sites with minimal diurnal variation once winds strengthened. Korea Meteorological Administration guidance and station data converged on this outcome, consistent with early-October climatology of 19–21°C averages after the seasonal transition. The only realistic challenges would involve a last-hour measurement revision at the specific Incheon or Seoul reference station or an unforecasted microscale warming event, both low-probability given the stable synoptic pattern and consensus model runs.