Numerical Weather Prediction (NWP) is acknowledged as being of vital importance to economy. The demand that NWP places on computing system performance has increased dramatically since the introduction of computer systems and is still growing. This paper describes the implementation of a one-level barotropic spectral NWP model using the high level functional language SISAL. Our initial approach was a direct transliteration from FORTRAN to SISAL, the result of which was a large amount of array copying and loss of parallelism. The algorithm of the model was then rearranged to enable effective exploitation of parallelism by an optimizing SISAL compiler. The parallel implementation technique and its potential for modeling with larger model sizes are discussed, and the improved results for an Encore Multimax multiprocessor are presented.