Ensemble forecasts from the Met Office and ECMWF center London’s maximum temperature for May 18 near 14–16 °C, aligning closely with the market’s leading outcomes at 15 °C and 14 °C. A broad Atlantic low-pressure system is driving scattered showers and moderate westerly winds across southern England, suppressing daytime insolation while increasing turbulent mixing that caps surface warming. Small variations in shower timing, cloud cover depth, or wind direction can readily shift the observed high by 1–2 °C, which accounts for the substantial probability assigned to adjacent outcomes. These short-range model spreads are typical for an unsettled mid-May pattern, with the next official guidance updates expected to refine the range before standard station observations are recorded.