Probability forecasts – Part 1: ensembles and probabilistic forecasts

Abstract Ensemble weather forecasts have been in use for many years to help forecasters understand uncertainty in weather predictions but are now becoming core to some operational NWP systems. Ensembles offer greater predictive skill and information content than deterministic forecasts but are more complex to interpret and communicate; essential if users are to benefit through improved decision‐making. This Part 1 introduces the rationale for ensemble forecasts and illustrates some of the ways they can be post‐processed to produce useful forecast information in support of decision‐making. Part 2 will consider effective communication of such information and ask the question, will people understand?