A fully-implicit semi-Lagrangian dynamic core is being developed at NCEP/EMC, as a part of the multi-institutional WRF initiative. The model is governed by the fully compressible, nonhydrostatic, moist diabatic equations of the atmosphere. A hydrostatic-pressure based, terrain-following, hybrid coordinate is used in the vertical. Equations are discretized on a 3D unstaggered grid. Use of this grid is anticipated to enhance the efficiency of the semi-Lagrangian computations. Use of a fully-implicit treatment of the gravity and acoustic wave terms, on the other hand, is anticipated to enhance the numerical stability of the model, beyond what is generally achieved in a traditional semi-implicit semi-Lagrangian model. Details of the model formulation and preliminary results will be presented at the conference.