Recent forecast guidance from the Japan Meteorological Agency places Tokyo’s May 18 overnight minimum near 18–19 °C, driven by a building high-pressure ridge that limits cloud cover and allows moderate radiative cooling under light northerly flow. This positions 18 °C as the slight market leader at 30 percent implied probability, with 19 °C and 17 °C tightly bunched because small shifts in boundary-layer moisture or the exact timing of any weak frontal passage can swing the low by one degree. Mid-May climatology supports this narrow band, with historical minima clustering between 15 °C and 19 °C as spring warmth gradually displaces cooler maritime air. Traders are monitoring the next model runs for any changes in dew-point forecasts or wind speed that could alter surface cooling rates before resolution.