Many observational and modeling studies suggested that the low-level circulation that supplies moisture from the tropics along the Gulf of California played an important role in determining the precipitation pattern over the southwestern North America. However, the structure of the low-level circulation and its associated moisture and energy budget remain largely unvalidated and incompletely understood. For instance, the amplitude of the diurnal cycle over the core North American monsoon region far exceeds the amplitude of the annual cycle, but it is unclear how the low-level circulation is related to the diurnal cycle of moisture and convection and how this relationship contributes to the location and spatial structure of precipitation. Among the main reasons for these are that the past numerical models cannot properly resolve Baja California and the Gulf of California and poorly represent the diurnal cycle and its related processes. As a consequence, models have difficulties in explaining the observed precipitation distribution and its timing. On the other hand, analyses of the low-level circulation features based on satellite remote sensing have accuracy deficiencies and thus require further calibration. In this study, we will investigate the structure and variability of the low-level circulation along the Gulf of California and west slopes of the Sierra Madre Occidental of the northwest Mexico and precipitation patterns over the core North American monsoon region, using the coupled MM5/SSiB model. A two-way, nested-grid (45/15 km) technique is employed to achieve multi-scale simulations. The QuikSCAT surface wind is assimilated into the MM5/SSiB using a Four-Dimensional Data Assimilation (FDDA) scheme to improve the simulation of the low-level circulation. A one-month continuous model simulation for June 2000 is performed to examine the model capabilities in capturing the low-level circulation and precipitation features. The simulation results will be analyzed to enhance our knowledge of the relationship between the diurnal cycle and precipitation patterns over the southwestern North America.