Recent forecasts from the Japan Meteorological Agency indicate Tokyo's overnight minimum on May 20 will likely fall between 17°C and 21°C, with ensemble models showing modest spread due to variable springtime cloud cover and possible light rain. These conditions typically moderate radiative cooling compared to clearer nights, keeping lows several degrees above early-May averages while preventing rapid warming toward 23°C or higher. Historical climatology places the seasonal baseline near 15–16°C, yet current steering patterns and sea-surface temperatures favor a slightly milder outcome. Traders price the 18°C outcome highest because it aligns with the central tendency of latest model runs, though a shift in the next 48-hour forecast could easily favor 19°C or 20°C.