A three-dimensional meteorological micro-scale model was combined with a forestry windthrow tool to study the risk of urban trees under stormy conditions. The numerical results for an isolated individual tree are compared to general findings from forestry and pulling experiments in the field. It was found that the model is suitable to capture the critical wind speed in the right order of magnitude which causes strong hazards to trees. The new tool was applied to a limited real city environment during storm "Xavier", where significant and extensive tree damage have been observed. A comparison of the numerical results with the observations demonstrates a very reasonable agreement not only for the onset of windthrow but also for the right areal distribution during different phases of the storm.