An intense orogenic squall line was observed over the southern flank of the Alps during the IOP 2A (17-18 September 1999) of the Mesoscale Alpine Programme field phase. Using data from the Doppler/Polarimetric radars of the Lago Maggiore Target Area, the four-dimensional dynamic, thermodynamic and microphysical structure of the mesoscale convective system is reconstructed. According to the morphology, intensity and location of the reflectivity echoes, three main stages are distinguished.: the initiation stage (1400-1700 UTC 17 September 1999) was characterized by a series of nearly stationary lines of weak to moderate convection parallel to the mid-level southwesterly flow. These lines were formed of short-lived cells triggered by some specific terrain features of the first slopes of the Alps to the north of the Lago Maggiore region. The subsequent gradual formation of a southwest-northeast oriented line of intense convection at the foothills of the alpine slopes is shown to result from a phasing between the low-level orographic forcing and a moving mid-level trough (phasing stage 1800-2100 UTC 17 September 1999). During the third stage (explosive stage 2100-2200 UTC 17 September 1999), the squall line evolved into a three-dimensional convective cluster as it continued its southeastward propagation over the moisture-rich Po plains. In each of these stages, the importance of the small-scale topographic features and of the associated dynamical processes (channeling in the valleys and blocking of the low-level flow) is demonstrated.