Sub-seasonal predictability of heavy precipitation–associated cyclones in the Sahara

Heavy precipitation events (HPEs) are a precious source of water in the Sahara, but they also trigger potentially devastating flooding. Saharan HPEs are strongly associated with surface cyclones, making accurate cyclone forecasting crucial for predicting hydrometeorological hazards and their impacts. In this study, we investigate the predictability of HPE-associated cyclones across the Sahara and its drivers. We use ERA5 reanalysis between December 2000 and November 2020 to evaluate ensemble ECMWF reforecasts and to identify the atmospheric conditions controlling forecast skill. Short-, medium-, and extended-range forecast skill is evaluated based on the overlapping areas of observed and forecasted cyclones over the Sahara. Results show that the lead time of skilful prediction is up to about 10 days. On short-range lead times, forecast skill is higher in winter, whereas on medium to extended lead times, skill is higher in summer and fall. In winter, when cyclones are mainly located in the northern Sahara, forecast skill is higher for deeper cyclones. In summer, skill is higher for cyclones located in the southwestern Sahara. Rossby wave patterns extending over the North Atlantic are associated with both high and low skill forecasts, highlighting a flow-dependent control on predictability over the Sahara and underscoring the need for more detailed investigation. These findings show key characteristics of skilful HPE-associated cyclone forecasts on timescales of a few days to two weeks in advance. Understanding these variations across regions and seasons is key to improving the predictability of HPEs and their related impacts.