Traders' overwhelming consensus on 18°C as Tokyo's highest temperature on March 19 stems from confirmed observations by the Japan Meteorological Agency (JMA), which recorded a peak of 18.0°C at central Tokyo stations amid mild spring conditions with light winds and partial cloud cover. Historical March data shows averages around 13°C, but recent mild air masses from the south pushed readings higher without exceeding 19°C, aligning with ensemble model forecasts from ECMWF and GFS that capped diurnal highs near 18°C. Realistic challenges include rare post hoc data revisions from peripheral sensors or microscale urban heat anomalies, though JMA's quality-controlled measurements render these improbable, solidifying the market's resolution.