Short-term forecast uncertainty for Tokyo’s overnight radiative cooling on July 16–17 is the dominant driver of the tightly bunched market odds around 25–28 °C. Current ensemble guidance shows modest differences in expected cloud cover, boundary-layer moisture, and weak southerly flow, all of which can shift the minimum by 1–2 °C. With no strong frontal passage or typhoon influence anticipated in the next 48 hours, the range reflects typical midsummer variability under a subtropical high, where urban heat retention and humidity limit cooling. Historical July minima average near 25 °C, providing a climatological anchor that traders appear to weigh alongside the latest model runs. Updated forecasts from the Japan Meteorological Agency over the next 24 hours are likely to sharpen the distribution.