Skill in nowcasting high-impact heavy precipitation events

The objective of this study is to assess the skill of a precipitation nowcasting (very short range forecasting) system, with particular emphasis on hig-impact Heavy Precipitation Events (HPE). A threshold amount of 20 mm in 30 minutes is adopted, according to the first warning level issued for high rainfall rates by the Meteorological Service of Catalonia (NE Spain). These type of events are relatively frequent during autumn in the Mediterranean area and may produce flash-floods and substantial socio-economic losses when affecting densely populated areas in small catchments near the coast. Moreover, they can also produce local damage due to severe weather induced by winds of convective origin such as tornadoes or microbursts. Therefore, under these circumstances nowcasting systems producing very-short range (<3h) quantitative precipitation forecasts (QPF) are particularly necessary. Recent studies have pointed out the benefits of combining or blending the information from radar-based nowcasting systems and high resolution Numerical Weather Prediction (NWP) models to fill the gap between the lead-times of forecasts provided by each type of system - see for example Atencia et al. (2010), Berenguer et al. (2012) or the review by Pierce et al. (2012). Here we focus on assessing the quality of precipitation forecasts to examine realistic lead-times provided in four selected cases (see Table 1) for different precipitation amounts. The evaluation is done in two ways. Firstly forecasts are compared against radar based Quantitative Precipitation Estimates (QPE). Secondly the QPF fields are compared against actual raingauge data. Results are presented for all events together and for individual cases.