Traders have converged on 13°C as the overwhelming favorite for London’s lowest temperature on June 14 because official Met Office and ECMWF ensemble guidance, combined with recent surface observations, consistently point to an overnight minimum near that value under clear skies and light easterly winds. Radiative cooling after daytime highs in the low 20s Celsius allows temperatures to fall efficiently once insolation ends, while a stable air mass limits advection of milder Atlantic air. Historical June averages at Heathrow of roughly 12–14°C provide climatological context that reinforces the current model consensus. A realistic shift away from 13°C would require unexpected increases in low-level cloud cover or wind speed that suppress radiative losses, or a faster arrival of a warmer front—developments that current short-range guidance assigns very low probability before midnight.