Mild southwesterly airflow from the Atlantic, combined with persistent cloud cover and elevated humidity, is anchoring market-implied odds near 18–19°C for London’s September 3 minimum. These conditions limit nocturnal radiative cooling under the urban heat island, where concrete and reduced wind speeds retain warmth compared with rural baselines. Official forecasts from the Met Office and ensemble models show overnight readings clustered in the upper teens Celsius, consistent with early-autumn climatology but elevated by the recent warm spell. Slight model differences in timing of any clearer intervals or wind shifts create the tight 43.5%–42% split between 19°C and 18°C, with lower probabilities assigned to 17°C or warmer outcomes if cooling strengthens more than expected.