A short-range numerical prediction model, which is part of a real-time 3-h data assimilation and forecast system, is described. The distinguishing feature of the model is the use of terrain-following (σ) coordinate surfaces in the lower troposphere combined with isentropic (θ) surfaces aloft. Such a hybrid coordinate system allows modeling of processes in a convectively unstable boundary layer while retaining tile advantages of θ coordinates in representing upper-tropospheric frontal and jet-stream structures. The hybrid approach used in this model represents a in major departure from previous hybrid formulations in atmospheric models, oven though it has been used for more than ten years in oceanic modeling. Part I of this two-part paper contains a thorough description of the model and the results of validation experiments. Results of North American case studies wig be reported in Part II.