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We present a new three-dimensional magnetohydrodynamic (MHD) model that describes the evolution of coronal magnetic arcades in response to photospheric Ñows. The dynamics of the system is discussed, and it is shown that the model reproduces a number of features characteristic of coronal mass ejection (CME) observations. In particular, we propose explanations for the three-part structure of CMEs observed at the solar limb and the formation and evolution of sigmoids and overlaying arcades. The model includes the e †ects of Ðnite resistivity in the system and morphological changes induced by reconnection are studied. Reconnection is found to prevent the formation of highly twisted magnetic structures if the magnitude of photospheric velocity is close to the observed values. In addition, we suggest a novel explanation for the splitting of the CMEÏs core prominence material observed in some eruptions. The distinguishing features of the model are the novel numerical methods used to evolve the MHD system and the formulation of the boundary conditions. The sti †ness of the resistive MHD equations constitutes a major difficulty for numerical simulations. Calculations in our model are performed using recently introduced exponential propagation techniques that allow efficient integration of the equations with time steps far exceeding the CFL bound that constrains explicit schemes.