**Official forecasts from the Hong Kong Observatory set the primary driver for trader sentiment on the September 21 maximum temperature.** The HKO outlook calls for mainly cloudy conditions with a few showers and isolated thunderstorms plus sunny intervals, producing a projected range of 27–31°C. This aligns with the market’s leading outcomes clustered tightly around 30–32°C (combined ~88% implied probability), where 31°C holds a modest edge at 36.5%. Cloud cover and convective activity are the key differentiating factors. Reduced solar insolation from increased cloudiness, combined with the timing and intensity of any showers or thunderstorms, can suppress peak heating by 1–2°C relative to clearer days. Northeast winds of force 2–3 and elevated humidity (70–95%) further moderate daytime warming. Recent days have seen hotter readings (up to 33°C under clearer skies), but the shift to more disturbed weather supports the current forecast consensus and explains why probabilities remain closely balanced rather than strongly favoring one value. Updated model runs and any last-minute changes in shower coverage remain the main near-term variables for traders.