Recent Met Office and ECMWF ensemble forecasts indicate London’s May 17 maximum will most likely reach 15–16 °C under moderate westerly flow that advects cooler Atlantic maritime air across southern England. Afternoon cloud cover and the precise timing of any weak frontal passages introduce the main uncertainty, with clearer skies potentially allowing brief warming to 17 °C while thicker cloud or earlier rain would favor the lower end. Mid-May climatological averages near Heathrow hover around 16–17 °C, so the tight clustering of market-implied probabilities between 15 °C and 16 °C reflects the narrow range of plausible outcomes once local boundary-layer mixing and insolation are accounted for. Updated 06Z and 12Z model runs on May 17 will provide the final guidance ahead of resolution.