**Trader sentiment for Tokyo’s June 20 minimum temperature centers on 20–22 °C because official short-range forecasts and the ongoing tsuyu rainy season point to a mild overnight low under persistent cloud cover and scattered showers.** Japan Meteorological Agency guidance and extended models show June 20 featuring cloudy skies with occasional rain, high around 24 °C, and minimum near 19–20 °C—conditions typical of mid-June when the diurnal range narrows as low-level moisture and clouds limit nighttime radiative cooling. Historical climatology supports this: average June lows in Tokyo hover around 18–21 °C, but cloudier, rainier episodes often push the actual minimum 1–2 °C higher than clear-sky nights. The tight clustering of probabilities (21 °C leading at 30 %, followed closely by 18–23 °C outcomes) reflects model uncertainty in exact cloud thickness, shower timing, and boundary-layer wind, all of which can shift the recorded low by a degree or two. Stronger or more prolonged rain could enhance evaporative cooling and favor the lower tail (18–19 °C), while thicker stratiform clouds would insulate and support 22–23 °C. Extreme outliers below 17 °C or above 25 °C remain negligible because no anomalous high-pressure or clear-sky regime is expected within the two-day window. Traders are therefore pricing the most probable outcome under standard tsuyu meteorology rather than any sudden shift in large-scale circulation.