A study of two oceanic extratropical cyclones, generated by the Navy Operational Atmospheric Prediction System (NOGAPS) model reveals a simultaneous development of the upper- and lower-tropospheric features while these features maintain their positions relative to each other. These cyclones also develop extreme shear in most synoptic fields after 120 hours of numerical simulation. The mass budgers of these cyclones show a concentration of inward lateral transport (convergence) in the lowest model layer and outward lateral transport (divergence) in a layer centered near 300 mb. Time sections of lateral transport, vertical velocity and mass tendency reveal that these cyclones develop in two phases--an explosive cyclogenesis phase, and a quasi-steady state phase. These time sections also reveal a 12-hour cyclic pattern embedded within the two phases of cyclone evolution. This 12-hour cyclic pattern is not evident in the map sequences of the surface pressure.