Recent JMA and ensemble forecasts position Tokyo’s August 20 maximum near 31–32 °C under mostly cloudy conditions with light showers possible, producing the tight clustering of market-implied odds around those values. Persistent summer humidity and the urban heat island effect support readings at or above the 31 °C climatological average, while increased cloud cover and modest precipitation reduce peak solar insolation enough to keep the outcome below 33 °C in most model runs. Small differences in timing of any showers or afternoon clearing, plus minor variations across official measurement sites, explain the spread among the 30–32 °C brackets; traders are weighting the consensus guidance that has held steady over the past 48 hours.