The purpose of this talk is to present a new generalized potential vorticity inversion method based on the Ertel formulation and an implicit balance. During the definition of the initial condition of a Numerical Weather Prediction model, the excitation of spurious gravity waves must be prevented. In the french model --ARPEGE-- the control of the gravity waves is performed by digital filtering. It is demonstrated how this implicit balance provided by time filtering can be used to invert the potential vorticity field in the frame of a primitive equation model. Particular attention is paid to the balanced divergent part of the flow, a direct outcome of the method, for various potential vorticity fields associated to two cases of strong cyclogenesis. Just after Christmas 1999, two exceptionally severe storms have crossed France and western Europe causing heavy damages and human losses. This paper presents a dynamical overview of the two storms using objective tools such as the computation of the ageostrophic flow associated to potential vorticity features thanks to the generalized approach of the inversion presented above. This study reveals that the propagation of the first storm before its main growing phase is driven by an upper level disturbance. However, the role of the jet outflow during the explosive growing phase over the western part of France is decisive. The second storm looked like a straightforward triggering of the cyclone by an upper tropospheric disturbance.