Recent ensemble forecasts from the Met Office and ECMWF have converged on a daytime maximum of 18°C for London on May 17 under partly cloudy skies and moderate westerly flow that limits afternoon heating. This model agreement, consistent with typical May climatology for the region, drives the market's near-certain pricing for the 18°C outcome. Official observations at central London stations generally track these guidance products closely on stable days, though minor urban heat island variations or unexpected cloud breaks remain the primary realistic scenarios that could shift the verified high. Traders should watch for any final Met Office updates as afternoon measurements finalize.