**Forecast models from the Japan Meteorological Agency and global ensembles indicate a minimum temperature in Tokyo on September 26 of 18–19°C, driven by a stationary frontal system and developing low pressure east of the Kanto region.** Persistent rain, northerly winds, and high humidity (near 90%) are suppressing daytime heating and promoting overnight cooling, with hourly guidance showing temperatures bottoming out near dawn under overcast skies. This aligns closely with market-implied odds favoring 18°C (53.5%) and 19°C (34.5%), while lower probabilities for 17°C or warmer outcomes reflect limited upside from any brief clearing. Historical September minima average around 18°C, providing additional context for the tight distribution. Updated model runs and official briefings overnight will offer the next key data points for resolution.