**Stable high-pressure conditions and clear skies are positioning 27°C as the leading outcome for Incheon/Seoul's daily maximum on September 20, 2026, with market-implied odds reflecting tight clustering around 26–28°C.** Official guidance from the Korea Meteorological Administration and multiple numerical weather models (including GFS and ECMWF runs) consistently forecast a high near 27°C under sunny to mostly clear conditions, light northeasterly to northerly winds of 5–10 km/h, and negligible precipitation risk. This aligns with the seasonal transition in late September, when the strengthening Siberian high promotes subsidence and drier air masses, moderating temperatures after summer while still allowing sufficient daytime insolation for highs in the mid-to-upper 20s°C. Recent 48-hour observations show stable surface pressures near 1020 hPa and dew points in the mid-teens Celsius, supporting efficient radiational cooling overnight but limited overnight recovery that keeps the diurnal range moderate. Model consensus shows low spread for peak values, with only minor variations tied to exact timing of maximum heating or boundary-layer mixing. Historical September climatology for Incheon places average daily maxima near 25–26°C, so the current setup represents slightly above-normal but not anomalous warmth. Traders appear to be weighting the official forecast heavily while allowing for small upside or downside revisions from the final model update before the 20th, explaining the combined ~98% probability mass on 25–28°C. No significant synoptic changes (such as frontal passages or typhoon influences) are expected in the next 48 hours that would shift the outlook.