Persistent northeasterly monsoon flow across northern Taiwan is the main driver positioning the 50% implied probability on a minimum of 20°C or below for Taipei on September 18. This pattern delivers cooler maritime air and scattered showers that enhance overnight radiative cooling, with Central Weather Administration guidance and multiple model runs converging on lows near 23–24°C amid partly cloudy to overcast conditions. Recent days have shown stable highs in the upper 20s Celsius but steadily declining overnight readings tied to the wind regime, which historically supports minima 1–2°C below seasonal averages during active northeast monsoon episodes. Updated forecast discussions and the next model cycle will provide key data points for traders assessing any further downward revisions.