Recent ensemble forecasts from the Japan Meteorological Agency highlight substantial uncertainty in overnight radiative cooling over Tokyo through May 21, driven by a strengthening high-pressure ridge interacting with a passing frontal boundary. This setup can produce clear skies that allow minimum temperatures to drop below 11 °C or maintain mild conditions near or above 21 °C, depending on exact wind patterns and moisture levels. Historical May climatology shows average lows near 16 °C, yet current model spreads reflect typical springtime variability in the East Asian monsoon transition. Traders are weighting these divergent temperature outcomes nearly equally, with fresh guidance expected from updated runs ahead of the date.