Japan Meteorological Agency forecasts have locked in an overnight low of exactly 18 °C for Tokyo on May 17, anchoring trader consensus at 99.7 % implied probability for that single outcome. A stable high-pressure ridge over the Kanto Plain drives southerly warm advection that suppresses radiative cooling through the night, keeping temperatures within a tight 1 °C envelope across successive model runs. This synoptic setup aligns closely with mid-May climatology for the region, where average minima typically range from 14 °C to 18 °C. The dominant risk remains a last-minute adjustment in numerical weather prediction guidance or minor observational variance at the official measurement site that could shift the recorded minimum by 1 °C.