Current forecast models from the Japan Meteorological Agency indicate an expected overnight minimum for Tokyo on May 21 in the 15–18 °C range, consistent with late-spring climatology under warm Pacific maritime air ahead of the Baiu front. This baseline supports roughly equal market-implied odds for extremes because ensemble spreads show potential for stronger northerly flow that could drop readings to 11 °C or below, or for clearer skies and southerly advection that could keep the low above 21 °C. Key variables include the precise timing of any frontal passage and nocturnal radiative cooling rates, both of which remain sensitive to small shifts in upper-level steering patterns over the next few days. Updated model runs and official guidance issued in the coming 48 hours will likely narrow this uncertainty ahead of market resolution.