A seamless blended multi-model ensemble approach to probabilistic medium-range weather pattern forecasts over the UK

Abstract

This paper describes a new seamless blended multi-model ensemble configuration of an existing probabilistic medium- to extended-range weather pattern forecasting tool (called Decider) run operationally at the Met Office. In its initial configuration, the tool calculated and presented probabilistic weather pattern forecast information for five individual ensemble forecasting systems, which varied in terms of their number of ensemble members, horizontal resolution, update frequencies and forecast lead time. This resulted in multiple forecasts for the same validity time which varied in terms of forecast skill depending on the lead time in question. This presented challenges for end-users (e.g., operational meteorologists) in terms of knowing which forecast output is best to use and at which lead time, as well as knowing what to do in situations where forecasts varied between ensembles. To get around these challenges, a new seamless blended multi-model ensemble configuration has been implemented operationally, comprising of output from five separate ensembles, and provides a single best forecast from day one out to day 45. Objective verification for a set of eight weather pattern groups covering forecasts initialized over a 6-year period (2017–2022) shows that the seamless blended multi-model ensemble forecasts are at least as good as, if not better than the best performing individual model.