Numerical weather prediction is among the oldest fields of computational science, and existed before the advent of electronic computing. Thanks to the performance of modern computers, the fidelity of weather simulations has reached a point where they are indispensible in weather forecasting, and thus have become one of the economically most impactful domains of computational science. Typically, the dynamical cores of models of weather simulations are grid based and memory bandwidth bound, thus performing poorly on modern X86 type processors. In this presentation, we will discuss a refactoring project of the COSMO code that implements a regional climate model used by several weather services and academic institutions worldwide. The dynamical core has been rewritten and is easily portable to multiple architectures, including GPU. The physics part of the code is being ported to GPU with OpenACC directives. Preliminary performance results for production scale problems will be presented. Other contributors to this research include Oliver Fuhrer, Swiss Federal Office of Meteorology and Climatology MeteoSwiss, Tobias Gysi and David Müller, Supercomputing Systems AG, Xavier Lapillonne, Center for Climate Systems Modeling, ETH Zurich, William Sawyer, Ugo Varetto, and Mauro Bianco, Swiss National Supercomputing Center.