Recent ensemble guidance from the Japan Meteorological Agency points to a mild overnight minimum in Tokyo on May 20 driven by a strengthening high-pressure ridge over the Kanto Plain. This setup favors clear to partly cloudy skies, light southerly flow, and limited radiative cooling, keeping the low in the 18–21°C range that currently commands the highest implied probabilities. Model spread arises mainly from uncertainty in exact cloud timing and boundary-layer moisture, which can shift the minimum by 1–2°C. Mid-May climatology places the long-term average near 15°C, so the current outlook reflects a modest warm anomaly. Traders are monitoring the next JMA model runs and regional surface observations for any tightening or widening of that temperature envelope.