We describe numerical aspects of a semi-Lagrangian non-hydrostatic model being developed for forecasting and research applications at very small scales as part of the Weather Research and Forecasting (WRF) project. The model is designed to accommodate high-order numerical operators in space with an adaptation of a third-order Runge-Kutta time integration that allows an implicit treatment of acoustic modes. Large scale models have demonstrated significant that benefits result from the adoption of hybrid theta-sigma coordinates in the vertical. We are investigating the use of a general family of vertical coordinates which include both hybrid and conventional sigma coordinates as options available through the selection of appropriate parameters of this family. We report on the problems and prospects related to this development in the context of a three-dimensional semi-implicit model.