Using a fine-scale (10--20 km) nested regional modeling system, synoptic variations in climatological summertime low-level wind fields over the Gulf of California and the southwestern United States are described. Under appropriate synoptic conditions, strong surge events can develop. These are characterized by low-level southerly flow over the entire Gulf of California with southerly winds extending into Arizona, California, and southern Nevada. Vertically, these southerly winds are present through the bottom 1--2 km of the atmosphere. Southerly flow is persistent throughout the day with some local diurnal cycling over the foothills of the Sierra Madre Occidental and northwestern Mexico. Under nonsurge conditions the simulated low-level winds have a significantly different geographic structure. Nighttime southerly flow is limited to the northeastern Gulf and small portions of southwestern Arizona. Flow over the central and southern Gulf is northerly with weak, variable winds over the foothills of the Sierra Madre Occidental. During the day, southerly winds are present over the central and northern Gulf of California, extending into southwestern Arizona; however, this southerly wind pattern does not support continuous flow from the mouth of the Gulf into northwestern Mexico. Instead, there is a westerly component associated with predominantly upslope flow over the foothills of the Sierra Madre Occidental and the Sonora Desert.