High pressure and a warm southerly airmass over southern England suppressed overnight radiative cooling, anchoring Met Office and ensemble model guidance for a London minimum near 17 °C at the official London City Airport station on May 26. This setup produced the market’s near-certain 17 °C outcome by limiting heat loss under clear skies and light winds typical of the current heatwave pattern. Historical May overnight lows in London average 8–12 °C, so the elevated baseline reflects the anomalous southerly flow rather than typical spring conditions. Only a late forecast shift introducing stronger winds or unexpected cloud cover could have altered the reading, though no such change materialized in the final model runs.