In this study a potential-vorticity (PV) streamer which crossed the Alps during the Special Observing Period of the Mesoscale Alpine Programme (MAP) on 6 November 1999 is analysed. In situ aircraft data, wind-profiler data, model output data from the French non-hydrostatic mesoscale model (Meso-NH), and European Centre for Medium-Range Weather Forecasts (ECMWF) analyses are used to describe the structure and evolution of the streamer. In addition lower-stratospheric humidity which was remotely sensed by a Differential Absorption Lidar (DIAL) installed on board the aircraft is analysed. The comparison of measured and simulated lower-stratospheric humidity data show good agreement in structure and magnitude. The data analysis also reveals that waves are generated by the orography and transmitted into the lower stratosphere where they modify the streamer's PV structure. The impact of orography and diabatic processes on the streamer's evolution is simulated with MesoNH in a series of simulations. These simulations demonstrate that the separate effects of orography and diabatic processes are significant and comparable in magnitude, and that their combined influence leads to the formation of a PV cut-off. As a synergetic effect it is found that PV filaments are generated north of the Alpine barrier. The influence of orography and diabatic processes reduces the lower-stratospheric and upper-tropospheric PV by about 25% above the Alps, compared with the situation where neither effect is present.