In this study, we present an example of the benefit that can be achieved from carefully designed manual modification of a numerical weather prediction analysis. The case to be investigated is the severe winter storm of 7–8 January 2005, affecting the North Sea and southern Scandinavia. Modifications of potential vorticity (PV) fields according to features in water vapour (WV) images are combined with information from singular vectors (SV) in an attempt to improve the initial state over data sparse regions west of the British Isles. The apparent mismatch between features in the WV image and the upper level PV anomalies in the numerical analysis is corrected, mainly at levels indicated as sensitive by the fastest growing SVs, in order to maximize the impact on the simulation. Model reruns, based on the inverted corrected PV fields, were then performed. The manual correction of PV fields led to a substantial improvement of the simulations of the storm. The PV modifications were carried out by a digital analysis system, implemented at the Norwegian Meteorological Institute. This system allows the PV modifications to be done interactively within an operational time limit.