The explosive development phase of an extratropical cyclone (ETC) is examined using output generated by the fifth generation PSU/NCAR Mesoscale Model (MM5). MM5 was used to simulate the Intensive Observation Period (IOP) 4 storm of 4-5 January 1989 from the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA). MM5 output was generated by a full-physics model run using a 79 X 139 grid with a 60km grid spacing. A diagnosis of the simulated ETC is performed using the Zwack-Okossi (Z-O) development equation to examine the forcing mechanisms influencing development. A second-order Shapiro filter is used to partition the terms in the Z-O equation into synoptic-scale and subsynoptic-scale contributions to the near-surface synoptic-scale geostrophic vorticity tendency. The results presented here confirm previous studies that showed synoptic-scale vorticity advection, temperature advection, diabatic heating, and adiabatic cooling as important contributors to cyclone evolution. In addition, this study shows three additional terms to be significant: subsynoptic-scale diabatic heating, interaction of synoptic-scale static stability with subsynoptic-scale vertical motion (adiabatic term), and synoptic-scale ageostrophic vorticity tendency. These results demonstrate that even at a fairly coarse resolution of 60km subsynoptic-scale processes can be diagnosed as playing a significant role in ETC development.