Rainfall nowcasting: predictability of short-term extremes in Sweden

Abstract

Our current knowledge of the character of rainfall events in Sweden associated with extreme short-term accumulations and their predictability by forecasting, is very limited. In this study, observations from automatic stations and weather radars in Sweden were analysed to identify and characterise extreme short-term events. Often shorter-duration (1–6 h) extreme events were associated with small-scale structures, dominated by single cells, and longer-duration (12–24 h) events with less variable, larger-scale fields. For lead time 3 h, ∼20% of the events were forecasted at the correct place with an error of < 25% by the operational Swedish nowcasting system. If allowing for a 25 km displacement of the forecasted events, the hit rate increased by 10–15 percentage points. Some predictability was found for lead time 8 h but not for 24 h. The results suggest a potential added gain of increasing the temporal resolution of the Swedish flood forecasting system to sub-daily steps.