Stable high pressure and clear to partly cloudy skies over London are supporting peak daytime heating near the long-term September average of 21.2 °C at Heathrow, with Met Office guidance centering on a 22 °C maximum under light westerly flow and low humidity. This setup creates tight uncertainty between 22 °C and 23 °C because modest differences in afternoon cloud development or boundary-layer mixing can shift the daily high by 1 °C. Recent model runs show minimal precipitation risk and steady insolation, keeping probabilities balanced while limiting upside to 24 °C or higher. Official observations through midday confirm temperatures climbing steadily toward these thresholds, with final resolution hinging on the precise timing of peak solar heating this afternoon.