A model describing the propagation of buoyancy-driven flames and accelerated jet flames in a multicompartment building has been developed for lumped-parameter containment analysis codes. The model mimics the growth of flame fronts as observed from flame visualization experiments at Pisa University and captures the jet ignition phenomena observed in experiments at the Battelle Model Containment. The model establishes a complete scheme of flame propagation consisting of five flame modes, a fireball, a bubble, a prism, a spherical jet, and a planar jet. Through a flame transformation algorithm, flame propagation in a multicompartment system can be described by a birth and rebirth of these flame modes as many times as necessary until burning is complete. The model was implemented into the MAAP4 code. Comparison of the model prediction with Battelle's hydrogen test data (test H5) shows good agreement between the model and the experiment. The model correctly predicts the timing of jet ignition and the magnitude of pressure loads in the downstream compartment. The model was developed for the analysis of hydrogen deflagrations in any compartmentalized building including a reactor containment.