A small wave on the polar front crossed the Atlantic and experienced strong explosive cyclogenesis on 28 March 1997 when passing over the North Sea. It then progressed eastwards over northern Poland causing severe damage and the death of at least 11 people. A phenomenological study of the event is presented based on a potential vorticity (PV) approach supplemented by a synoptic analysis. Results confirm previous findings about the impact of a dry stratospheric intrusion on the process of explosive cyclogenesis and fit well with the conceptual model described by Browning (1997) concerning diffluent flow cyclogenesis. Also apparent is the role of potential vorticity advection, which is associated with the rapid change of flow pattern in the upper atmosphere. The careful analysis of potential vorticity fields every 100 m suggests that further research is required into fine mesh analysis and wind field retrieval.