A non-hydrostatic semi-Lagrangian dynamical core is under development as part of the multi-institution Weather Research and Forecasting (WRF) initiative. The model employs high-order operators for spatial differencing and for the mass-conserving grid-to-grid `cascade' interpolations. The semi-Lagrangian advection with forward trajectories permits the implementation of low-storage third- and fourth-order Runge-Kutta time integrations. Suitable modifications of the Runge-Kutta scheme allow the fast acoustic modes of the model to be treated in a semi-implicit way. We shall describe some of these features in greater detail and discuss implementation strategies that take advantage of massive parallelization. Results from preliminary tests will also be presented.