The integrated Forecasting System (IFS) of the European Centre for Medium-range Weather Forecasts (ECMWF) is a spectral weather forecasting model, which daily produces weather forecasts on up to 16 processors of a CRAY C90. This paper describes the shared-memory implementation of the code and the subsequent development that has been carried out in order to generate a parallel version, suitable for a scalable distributed-memory architecture with many processors. Performance results presented for several vector and parallel systems indicate that the parallelization effort has been successful in achieving good performance and high efficiency.