This paper presents the second phase of a continuing study on urban stormwater modeling. Building upon the detailed Civil 3D design developed in the first paper, including the accurate pavement detention volume and complete pipe network, the full drainage system is modeled in Autodesk Storm and Sanitary Analysis (SSA) to obtain the true hydraulic outflow based on both the geometric storage and the actual pipe flow behavior. The outflow results from SSA are compared with those computed using the Rational Method to evaluate differences between simplified hydrologic estimations and detailed hydraulic simulation. The same drainage configuration is also modeled in HEC-HMS to review the hydrologic response under a 100-year, 24-hour design storm and to provide an additional layer of model comparison. Furthermore, a new Python script is introduced that automatically processes SSA output files to generate a precise stage–storage curve for the underground detention system and reports the corresponding water surface elevation for any selected storage volume. This integrated workflow offers a comprehensive evaluation of stormwater performance using CAD-based geometry, hydraulic simulation, hydrologic modeling, and automated volume computation.