Systematic quality assessment of an operational seasonal forecasting system

Seasonal forecasts are increasingly taken into account in the decision processes of various sectors. The predictability of the climate several months ahead varies strongly in space and time. Detailed information on the quality of the forecasts is compulsory to make optimal use of the available forecasts. Such information, however, is not readily available. We have compiled a comprehensive validation of forecasts by an operational seasonal forecasting system, the ECMWF System 4. The forecasts for temperature and precipitation are verified against the ERA-Interim reanalysis, a proxy for observations, using a set of skill metrics describing various aspects of forecast quality. This report documents the data and methodology used and illustrates the variability of seasonal forecast skill by presenting results for selected seasons and regions of the world. To explore the space-time variability of forecast skill and to engage with users, we have developed an interactive web-app. This lowthreshold tool allows stakeholders to assess the quality of seasonal forecasts specifically for their region, season, forecast horizon and parameter of interest.