The size of a time step is important for numerical weather prediction models (NWP) since forecasts need to be available within the fraction of time that may considered to be valid. However, time step size is often limited by the numerical stability of the used advection schemes. Available schemes include semi-implicit Eulerian and semi-Lagrangian schemes. In principal, semi-Lagrangian formulations result in irregular communications on parallel architectures. In this paper we describe automatic code generation for a semi-implicit scheme with a semi-Lagrangian formulation. We describe how code can be generated from a mathematical specification of the advection model and we show results from preliminary experiments we have conducted with the generated code and the reference code from a production NWP on a number of different architectures.