**Current JMA guidance and ensemble model runs place Tokyo’s overnight low for June 18 near 19–21 °C, producing the tight 42 % / 38 % split between the 21 °C and 20 °C contracts.** Mid-June climatology shows average minima rising from ~17 °C early in the month toward 19–20 °C by mid-month at the Otemachi station, so the market reflects this seasonal ramp plus short-term variability. Key differentiating variables include nocturnal cloud cover (which limits radiative cooling), boundary-layer moisture tied to the advancing Baiu front, light wind speeds that reduce mixing, and the urban heat-island effect that adds 1–2 °C relative to rural sites. Recent model updates have narrowed the spread around 20–21 °C while still allowing a small tail probability for 19 °C or lower if clearer skies develop. Updated JMA and global model runs expected in the next 24–48 hours will likely resolve most of the remaining uncertainty before market resolution.