A case of deep and rapid cyclogenesis in the lee of the Alps is analysed by means of cross-sections, isentropic maps and trajectories based on synoptic data, to investigate the three-dimensional structure of the phenomenon and the nature of the processes that are responsible for it. The results of the analysis are also compared with available theoretical models. Considerations of the various scales involved in the development lead to a description of the phenomenon in terms of two distinct phases: a very rapid ‘trigger’ phase due to interaction between the frontal layer and the Alps, and a more usual ‘baroclinic development’ phase. Through a selection process typical of baroclinic instability, the lee cyclone acquires the observed horizontal and vertical scales and undergoes the normal life of a mid-latitude depression. The insufficiency of the present synoptic network for a satisfactory analysis of such a meteorological phenomenon is also stressed.