Forecast uncertainty from numerical weather prediction models and afternoon convective showers drives the tight clustering of implied probabilities around 28–30°C for the August 17 maximum in Seoul/Incheon. The Korea Meteorological Administration and ensemble guidance show a mostly cloudy regime with 20–60% precipitation chances that could cap daytime heating through evaporative cooling and reduced insolation, while calm steering flow and high boundary-layer humidity favor peaks near 30°C if showers remain scattered. Incheon’s coastal location typically runs 1–2°C cooler than central Seoul’s urban heat island, adding resolution variance. Historical mid-August climatology places the mean daily maximum near 30°C, but recent model runs have trended slightly cooler with increased moisture. Updated 00Z and 12Z guidance plus radar trends tomorrow morning represent the next key inputs for traders.