Recent numerical weather prediction runs from the Japan Meteorological Agency and global ensembles show a narrow spread in overnight minimum temperatures for Tokyo on May 21, with most guidance clustering between 14°C and 18°C as the city remains under warm Pacific maritime air ahead of the Baiu front. This transitional late-spring pattern historically produces average lows near 16°C, yet small shifts in cloud cover, wind direction, or the timing of frontal passage can easily alter the exact minimum by 1–2°C. With no single outcome exceeding 15% implied probability, traders are pricing the inherent uncertainty in 48–72-hour model solutions and the potential for revised guidance from the next JMA update cycle.