Official forecasts from the UK Met Office and supporting models indicate a maximum of 21°C in London on September 26, following overnight rain clearance and the development of fine, dry conditions with light winds and sunny spells under building high pressure. Afternoon temperatures are expected to peak near 20–21°C amid partly cloudy skies, consistent with late-September climatology where highs average around 18–20°C. Minor model spread in cloud timing and boundary-layer mixing creates tight uncertainty between 20°C and 21°C resolution thresholds, while stronger insolation or delayed clearing could briefly push readings to 22°C. Traders weigh these near-term forecast details against historical variability in maximum temperature observations at the primary reporting station.