How well do Met Office post‐processed site‐specific probabilistic forecasts predict relative‐extreme events?

The Met Office routinely generates post‐processed forecasts at sites throughout the United Kingdom; both deterministic and probabilistic products exist and deterministic data populate the publicly available website. In recent years, providers of weather information have focused upon the impact of events; impact is often related to the frequency of occurrence of an event at a site which is determined by its climatology. The ability with which a site‐specific forecast predicts relative‐extremes may be investigated by examining the skill with which these events (defined in terms of a percentile chosen from the climatology at each site) are predicted. The blended, deterministic, website forecast is less likely to forecast extreme events; therefore, the probabilistic forecast product (which does not currently appear on the Met Office website) was evaluated for its ability to predict heavy rainfall (RF 24 ), maximum summer day time temperature ( T max ), minimum winter night time temperature ( T min ) and strong winds (WS hrly ) over a 21 month period between December 2013 and August 2015. To this end, four methods of verification are considered: the Symmetric Extremal Dependency Index (SEDI), a threshold weighted version of the continuous ranked probability skill score (CRPSS) and a conditioned version of the CRPSS together with an analysis of the discrimination and reliability. Each method indicates forecast skill, with T max and RF 24 identified as the most and least skilful respectively and WS hrly identified as the most reliable. Site‐specific values of both versions of the CRPSS appear relatively well correlated and these scores also show correlation with SEDI for WS hrly .