Quantitative river flow forecasts for the Flood Outlook: high flow forecasting over the UK winter

•The Flood Forecasting Centre (FFC) commissioned UKCEH and the Met Office to explore the potential to improve assessments of flood risk at a sub-monthly timescale up to 4 weeks ahead. The aim is to enhance the skill and effectiveness of the Flood Outlook, a regular publication issued twice a month by the FFC, providing flood likelihood guidance over the next 6 to 30 days across England and Wales. •In support of this goal, the Met Office have developed an ensemble of analogue weekly meteorological forecasts that can be used to drive hydrological models. UKCEH have used these as input to our Grid-to-Grid hydrological model to generate river flow forecasts for the next 1 to 4 weeks ahead. Here, we assess the performance of these forecasts using a variety of statistical measures. These evaluate whether such a forecasting scheme would have been skilful if it were to have been operational over the winters (December, January, and February) of 1993-2016. •We conclude that river flow forecasts can be considered skilful over Great Britain at up to three weeks ahead and at up to four weeks ahead in some regions. This analysis offers strong support for the development of a sub-monthly operational high flow forecasting scheme to support the work of the Flood Forecasting Centre.