Stormwater systems, while designed to operate in free surface flow conditions, can undergo transition to pressurized flows during rapid filling conditions where inflows exceed the conveyance capacity of sewers and tunnels. Such conditions are referred as Mixed Flows or Flow Regime Transitions, and pose a challenge to traditional unsteady numerical models. The Storm Water Management Model (SWMM) model has an unsteady hydraulic solution algorithm based on EXTRAN module that can simulate mixed flows. However, due to its link-node solution procedure, SWMM has limited ability of representing highly dynamic flow conditions involving rapid filling of closed conduits and mixed flows. This work explores the accuracy of SWMM in representing three different experimental conditions involving mixed flows, testing effects of spatial and temporal discretization to the results. The original recommendation presented in SWMM literature is revised, and updated recommendations are proposed to improve SWMM simulation results in conditions involving mixed flows and rapid filling conditions.