**Northeast monsoon flow is the dominant factor keeping Taipei's September 18 daytime maximum in the narrow 29–32 °C range favored by traders.** Persistent northeasterly winds advect cooler, moister maritime air across northern Taiwan, limiting solar heating and producing scattered showers or drizzle that further cap temperatures. Official Central Weather Administration guidance and multiple numerical models converge on highs of 29–31 °C, with isolated guidance reaching 32 °C under brief clearing; this distribution directly explains the market’s heavy weighting toward 31 °C (49.5 %) and the secondary probabilities at 30 °C and 32 °C. No tropical systems or abrupt synoptic shifts are indicated in the latest runs, so the current consensus is expected to hold unless the monsoon weakens more rapidly than anticipated before peak heating.