**Market-implied odds cluster tightly around 19–21 °C for London’s daily minimum on 15 August 2026, reflecting genuine forecast uncertainty two days ahead amid a late-summer heatwave.** Current Met Office guidance shows daytime maxima falling from 33–35 °C on 13–14 August to 28 °C on the 15th, with an overnight low near 16 °C, yet ensemble spreads and rapid model updates keep the precise minimum fluid. The dominant driver is the transition from persistent high pressure and clear, humid nights—favoring limited radiative cooling and urban-heat-island retention that can hold minima in the 19–21 °C range—to possible increased cloud or a subtle Atlantic flow that would allow slightly more cooling toward 18–19 °C. Key differentiating variables include boundary-layer moisture, low-level wind speeds, and the exact timing of any weakening ridge; small shifts in these can swing the minimum by 1–2 °C. With resolution only 48 hours away, the next Met Office and ECMWF runs will likely tighten probabilities around the eventual observed value from official London stations.