**Recent forecasts position the overnight low for Seoul/Incheon on September 18 around 19–21°C, aligning closely with the market’s leading outcomes (20°C at 43.5%, 19°C at 30.5%, 18°C at 13.5%, and 21°C at 10.5%).** Multiple models and official-style guidance issued on September 17 show a stable high-pressure regime with light easterly to northeasterly winds, partly cloudy skies, and minimal precipitation. These conditions support moderate nocturnal radiative cooling after daytime highs near 27–29°C. Incheon’s coastal location and the prevailing flow from the warmer Yellow Sea limit deeper cooling, keeping minimums from dropping below the upper teens. Historical September climatology for the area places typical overnight lows in the 18–20°C range during this period, providing a baseline that current guidance largely follows without major deviations. No significant frontal passages or typhoon influences are indicated in the short-term window, so forecast spreads remain narrow. Traders appear to weight the consensus cluster around 19–20°C most heavily, with lower probabilities assigned to cooler outliers that would require clearer skies or a sharper wind shift than currently modeled. Updated model runs or Korean Meteorological Administration briefings on the 18th could refine the exact minimum before market resolution.