The closely matched probabilities across 18–21°C reflect substantial uncertainty in short-term ensemble forecasts for Tokyo’s overnight minimum on May 20. With only four days until resolution, Japan Meteorological Agency and global models continue to show notable spread driven by variable cloud cover, frontal passages, and humidity levels typical of late spring in the region. Historical climatology places average May lows near 15–18°C, yet current steering patterns suggest a modest warm bias if high pressure builds. Traders are watching the next 12–24-hour model updates for tighter consensus on wind and radiative cooling overnight, which will determine whether the low settles nearer the upper or lower end of the current distribution.