Recent Met Office and ensemble model guidance shows a building high-pressure ridge with northerly flow advecting cooler maritime air into southern England, limiting daytime warming on August 22. This setup supports a peak of 21–22 °C under partly cloudy skies and light winds, consistent with the market’s tight 41 % / 32 % split between those two outcomes. Minor model spread in boundary-layer mixing and cloud timing creates the narrow gap, while 23 °C odds remain lower because stronger insolation or a slight wind shift would be required to exceed recent observations of 20–21 °C. Traders are weighting the official London-area maximum, where northerly advection and modest August climatology anchor probabilities near the seasonal average.