High swell events can develop far from the coast under cyclonic conditions, and take several days to travel to land. If early warnings are not issued, they can take an area by surprise and have a devastating impact. This lesson aims to improve the ability of marine forecasters to forecast extreme marine events related to high swells. It does so by providing background information on winds and waves, and presenting a process for monitoring and forecasting high swell events using a variety of data. These include ASCAT scatterometer wind data and the ECMWF Extreme Forecast Index (EFI) product, which helps verify model output and improve the quality of heavy swell forecasts. The forecast process is applied to two cases that occurred on the Moroccan Atlantic coast in 2014. Note that the lesson has been developed with funding from EUMETSAT for the ASMET project.