Northeast monsoon flow over the South China coast is the dominant driver behind current market-implied odds clustered around 25–27 °C for Hong Kong’s official minimum temperature on September 12. Recent Hong Kong Observatory guidance shows a 25 °C forecast low amid increasing northeasterly winds of 4–6 on the Beaufort scale, higher humidity (60–95 %), and a rising chance of showers and thunderstorms that can limit overnight radiative cooling. Day-to-night temperature ranges are narrowing as cloud cover and moisture increase, while model consensus on monsoon strength and shower timing creates the narrow spread among 25 °C, 26 °C, and 27 °C outcomes. Traders are weighing these variables against historical September minima near 25–26 °C under similar monsoon regimes, with any shift in shower timing or wind speed likely to move probabilities before the next forecast update.