Persistent showers and high relative humidity tied to a broad low-pressure trough over the northern South China Sea are capping radiative heating and driving the tight market clustering around 30–31°C for Hong Kong’s August 24 maximum. Official guidance highlights mainly cloudy skies with frequent rain, which reduces solar insolation and favors evaporative cooling under light southwesterly flow. Model spread in rainfall timing and intensity creates the narrow gap between the 31°C and 30°C outcomes, while any brief sunny breaks could briefly push readings to 32°C and sustained downpours could limit peaks near 29°C. August climatology places the baseline near 31°C, but the active monsoon pattern adds downside risk to the upper outcomes.