Recent Japan Meteorological Agency guidance and ensemble model runs indicate an overnight low near 18 °C for Tokyo on May 18 under mostly cloudy conditions that reduce radiative cooling. This narrow central forecast, combined with typical 1–2 °C error margins at two-day lead times, accounts for the tight clustering of implied probabilities around 17 °C, 18 °C, and 19 °C. Key differentiating factors include variable low-level wind speeds that either enhance or suppress cooling, the precise timing of any scattered showers, and the urban heat-island effect measured at the official Otemachi station. Historical mid-May climatology shows mean minima of 15–17 °C, but the current synoptic pattern favors slightly milder readings, leaving traders balanced across the one-degree uncertainty band until the next model update.