Using a fine-scale nested regional modeling system, the diurnal forcing of summertime low-level winds over the Gulf of California and northwestern Mexico is investigated. On diurnal timescales, simulated nocturnal low-level jets develop over the northern portion of the Gulf, the foothills of the Sierra Madre Occidental, and parts of southern Arizona. The southerly component of the nocturnal jet is the result of a geostrophic balance involving the Coriolis force and a cross-gulf pressure gradient force associated with nighttime slope cooling over the elevated Sierra Madre Occidental. Additionally, horizontal temperature gradients over the sloped orography produce vertical variations in this cross-gulf pressure gradient force, generating the jet-like vertical shear in wind components above the nocturnal boundary layer; frictional effects are responsible for producing shear in the wind profiles below the boundary layer. This balance is distinctly different from the inertial balance that is believed to be responsible for the low-level jet over the Great Plains region of the United States. Daytime winds are part of a directly driven wind field forced by horizontal pressure gradients associated with slope heating (up the Sierra Madre Occidental) and sea-land temperature gradients (north of the Gulf). During synoptically forced surge events, a similar diurnal cycle in low-level flow is still present; however, the local thermal forcing appears to be superimposed upon the large-scale synoptic forcing, resulting in weaker up-slope flow during the day and stronger along-slope flow at night. ¿ 2001 American Geophysical Union