Improved seasonal hydrological forecasting for Great Britain

Abstract. Great Britain’s variable maritime climate has until relatively recently limited the utility of seasonal hydrological forecasts. The latest generations of seasonal atmospheric forecasting systems have created new opportunities to improve flow forecasting across Great Britain, such as for the UK Hydrological Outlook. Here, newly-developed high-resolution rainfall forecasts derived from historical weather analogues (HWA) conditioned on large-scale circulation patterns are used to drive a monthly-resolution national-scale hydrological model. We use rainfall hindcasts from 1993–2016 to evaluate the performance of these flow forecasts and demonstrate their skill, particularly for the UK winter. We show that the high performance of the rainfall forecasts is spatially complementary to the skill provided by hydrological memory in groundwater-dominated catchments. Our analyses pinpoint the regions which would benefit most from future improvements in the rainfall forecasting or hydrological modelling systems. The introduction of these rainfall forecasts now enables hydrological forecasting at unprecedented levels of detail across Great Britain and is a model that may be similarly beneficial elsewhere in the world.