The city of Graz is located in a basin southeast of the main ridge of the Alps. In wintertime during the night a weak southerly wind up to 100m above ground level (a.g.l.) above which a relatively strong northwesterly low-level jet develops. Different authors have reported on this phenomenon but a sound physical explanation has still not been found. During the dispersion of atmospheric trace elements – Graz project (DATE) this wind shear has also been observed by the use of hourly tethered balloon soundings. Here a case study is presented performed with the Graz mesoscale model (GRAMM), where the main features of the observed complex flow field were simulated. We hypothesised that the observed wind shear has its origin in a flow separation, where a part of the nocturnal, northwesterly out-valley wind flows over the hill range located west of the city of Graz, and the rest flows around it. This hypothesis was tested with the help of the numerical simulations. Furthermore, sensitivity studies were made, which revealed that the choice of the initial “background” wind direction and the internal Froude number in the model exerted a strong influence on the flow field in the city area.