**Official forecasts position 27°C as the most probable minimum temperature for Hong Kong on September 21, 2026.** The Hong Kong Observatory (HKO) 9-day forecast, updated September 19, projects a daily range of 27–31°C under mainly cloudy conditions with a few showers and isolated thunderstorms, consistent with light northeast winds and high relative humidity (70–95%). Multiple international models align closely, showing overnight lows near 26–27°C (roughly 80°F), influenced by persistent cloud cover that limits nocturnal radiative cooling and typical late-September subtropical patterns. Recent developments reinforce this outlook: an anticyclone aloft has maintained fine to partly cloudy conditions through September 19–20, with minimal cooling from light easterly flow and dew points around 24–25°C. No strong cold surge or significant frontal passage is expected in the next 48 hours, keeping minimums in the narrow 26–28°C band. Ensemble spreads and hourly guidance indicate modest downside risk to 25–26°C if heavier overnight showers enhance evaporative cooling, but upside potential above 28°C appears limited by the marine influence and humidity. Traders are weighting the HKO threshold heavily, as resolution relies on official Observatory observations, with the current 45.5% implied probability on 27°C reflecting consensus around this forecast low.