Recent short-range ensemble forecasts from the Japan Meteorological Agency and international models show a tight but uncertain spread for Tokyo’s overnight minimum on May 18, placing the most likely values between 16 °C and 19 °C. This clustering arises because a weak high-pressure ridge currently over the Kanto Plain is expected to allow moderate radiative cooling under partly cloudy skies and light northerly winds, yet small differences in cloud timing or a passing frontal boundary can shift the low by 1–2 °C. Historical May climatology at the Haneda station centers near 15–16 °C, so traders are weighting both the seasonal baseline and the latest 48-hour model runs that converge on near-normal conditions while acknowledging typical spring variability in nocturnal temperature. Final JMA observations will resolve the market within hours of sunrise on the 19th.