**Persistent frontal rain and cloud cover over Tokyo are anchoring trader expectations for a September 7 overnight low near 22–23 °C.** Official Japan Meteorological Agency guidance and multiple numerical models (including ECMWF and GFS runs referenced in public forecasts) show a stationary front bringing widespread showers and thunderstorms through Monday, with southeasterly flow and high humidity (90–97 %). These conditions limit nocturnal radiative cooling, preventing temperatures from dropping much below the mid-20s °C while also capping daytime heating. Current 10-day forecasts consistently place the Monday minimum between 21–24 °C, with several sources clustering around 22–23 °C for the pre-dawn hours. This scientific consensus—supported by observed surface observations showing recent overnight lows near 20–22 °C amid similar moisture—explains why the market assigns the highest implied probabilities to 23 °C (39.5 %) and 22 °C (28.0 %), followed by 21 °C (14.5 %). Lower probabilities for 19 °C or below reflect the insulating effect of cloud and rain, while the small tail for 25 °C+ aligns with scenarios of delayed clearing or warmer advection that remain unlikely under the current pattern. Upcoming JMA forecast updates and the next model cycles will be the key near-term catalysts, as any shift in frontal timing or intensity could adjust the minimum by 1–2 °C. Historical September climatology (average overnight lows ~21–22 °C) provides additional context that traders appear to be weighting alongside the active weather pattern.