Predictability of cyclones associated with heavy precipitation events in the Sahara

Abstract

Heavy precipitation events (HPEs) are a precious source of water in the Sahara, but often trigger devastating flooding. These events are strongly associated with surface cyclones, making accurate cyclone forecasting crucial for predicting hazards related to HPEs and their impacts. In this study, we investigate the predictability of HPE-associated cyclones across the Sahara and its drivers. We use ERA5 reanalysis and ECMWF reforecasts initialized between December 2000 and November 2020. Forecast skill on short-, medium-, and extended-range timescales is evaluated based on the overlapping areas of observed and forecasted cyclones over the Sahara. Results show that the lead time of skillful prediction is up to about 10 d. Forecast skill varies strongly with season. At short lead times, skill is higher in winter, whereas at medium to extended lead times, skill is relatively high in summer, albeit with increased false alarm rates. These seasonal differences are also reflected in cyclone location and characteristics: deeper northern Sahara cyclones are predicted better than shallower ones, while in summer, skillful forecasts are found mainly in the southwestern Sahara. Northern Saharan cyclones are better predicted when Rossby wave patterns are persistent, whereas transitions between circulation patterns correspond to reduced forecast skill. These findings suggest that the predictability of HPE-associated cyclones in the Sahara is flow-dependent, and that high predictive skill can extend to subseasonal timescales under favorable flow conditions. Understanding these variations across regions, seasons, and circulation patterns is key to improving the predictability of HPEs and their related impacts.