MM90: A scalable parallel implementation of the Penn State/NCAR Mesoscale Model (MM5) This paper describes MM90, a parallel regional weather model based on the Penn State/NCAR MM5. Parallelization of finite differencing, horizontal interpolation, and nesting on distributed-memory (message passing) computers is handled transparently using the RSL library package. Fortran90 modules, derived data types, dynamic memory allocation, pointers, and recursion are used, making the code modular, flexible, extensible, and run-time configurable. The model can dynamically sense and correct load imbalances. The paper provides performance, scaling, and load-balancing data collected on the IBM SP2 computers at Argonne National Laboratory and NASA Ames Laboratory. Future work will address the impact of parallel modifications on existing modeling software; an approach using commercially available source translation software is described.