Official forecasts from the Japan Meteorological Agency and ensemble models indicate a high near 21°C for Tokyo on September 30 under persistent cloud cover, scattered showers, and northeasterly flow advecting cooler maritime air. This suppresses daytime heating compared to seasonal normals around 24–25°C, with limited insolation and evaporative cooling from rainfall maintaining the cap. Trader consensus, reflected in the 40.5% implied probability for 21°C and combined 50%+ for 20–22°C, aligns closely with these runs; any late-model shift toward clearer skies or stronger southerly flow could nudge the outcome toward 22°C, though current guidance favors the lower end of the distribution.