**Persistent heavy rain and thunderstorms associated with a slow-moving weather system are the dominant drivers keeping Tokyo's overnight minimum temperatures clustered around 22–24°C on September 7.** Official forecasts from the Japan Meteorological Agency (JMA) and international models show widespread showers and cloud cover through the night, which limit radiative cooling and compress the diurnal temperature range. Southerly to southwesterly winds advect warm, humid maritime air (dew points near 23–25°C), further supporting minimums in the low-to-mid 20s rather than cooler values. Recent model consensus and current observations indicate the overnight low will most likely settle near 22–23°C, aligning with the market’s leading implied probabilities (23°C at 35.5%, 22°C at 24.5%, 24°C at 21.5%). Higher outcomes above 25°C remain improbable under continued precipitation and cloud cover, while probabilities below 21°C are discounted because the same moist, overcast conditions suppress stronger cooling. Historical September climatology for Tokyo (normal minimum near 21.7°C) provides context, but the active weather pattern this week is elevating the floor relative to clearer-sky baselines. Updated JMA and numerical guidance over the next 12–24 hours will be the key data releases that could shift these market-implied odds.