Quantitative precipitation forecasting in the UK

The goal of realistic precipitation forecasts has been a major driver of improvements to Numerical Weather Prediction (NWP). Increased computer power has enabled dramatic improvements in resolution, data assimilation and cloud physics amongst others. However, the impact of precipitation on all aspects of life is such that the results still fall short of what is required. In this paper I outline some of the main requirements for precipitation forecasts in the UK, identify the critical areas where NWP products fall short, and illustrate techniques being developed to address them. For very short lead times, high spatial and temporal resolution is required, especially for prediction of flash floods. To meet this need, the Met. Officeʹs Nimrod system generates nowcasts by integrating radar, satellite and rain gauge data with NWP products. At lead times of a day or so, requirements range from very light precipitation, especially in freezing conditions, to widespread frontal rain and thunderstorms. The mesoscale version of the Met. Officeʹs Unified NWP Model provides skilful forecasts of frontal precipitation structure and orographic influence at this range. However, an enhanced diagnosis technique provides additional information on convective storm probability. In the medium range, the focus is on risk reduction, via early warnings of adverse weather. Deterministic NWP forecasts are not reliable enough and a probabilistic approach is required. This is being addressed through processing of products from the ECMWF Ensemble Prediction System. Work is underway to interface these products to an automated warnings generator within the Met. Officeʹs Horace forecaster support system. Together, these developments are meeting many of the demands for precipitation forecasts that are currently unachievable with NWP models.