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.

Keywords::

• precipitation
• runoff
• flood forecasting

Acknowledgements

We thank Johan Södling, Karin Nyström, Günther Haase, Lars Norin and Sian de Koster for valuable assistance with various data and manuscript issues as well as two anonymous reviewers for many insightful and constructive comments.

Additional information

Funding

The study was mainly funded by the Swedish Civil Contingencies Agency (MSB), through project grant no. 2011-3774, with additional support from the Swedish Research Council Formas.