Mesoscale numerical model simulations were examined to investigate the thermal and topographic forcing of a boundary layer jet along the northern California coast from 0000 UTC May 2 to 1200 UTC May 3, 1990. Routine surface and upper air observations, satellite, and higher-resolution coastal observations (R/V Point Sur and Doppler wind profiler data) documented the diurnal evolution of a coastal marine atmospheric boundary layer (MABL), coastal cloud structure, and mesoscale features such as the coastal jet and land-sea breeze circulations that were reasonably well forecast by the numerical model. Sensitivity model simulations to coastal baroclinicity showed that the diurnal development and intensification of the coastal MABL jet located north of the San Francisco Bay area was a result of thermal forcing due to differential land-sea heating. Simulations in which the coastal baroclinicity was held fixed in time at a minimal strength showed no diurnal development of the MABL jet. The strength of the jet was approximated well by the thermal wind relationship, with the jet core located in the sloping marine inversion layer within one Rossby radius of deformation from the coastline. The steepness of the marine inversion at the coastline was enhanced by regions of strong MABL divergence just offshore and strong convergence onshore resulting from the sea breeze circulation. Modifications to coastal terrain in two separate sensitivity model simulations illustrated that the primary effect of coastal topography was to act as a barrier to the onshore intrusion of higher momentum offshore air. This blocking was due to a topographically forced vertical circulation with a strong descending branch located at the coast. This downslope flow and adiabatic warming steepened the MABL inversion at the coast and enhanced the coastal baroclinicity and the coastal jet. Simulations without coastal terrain showed a jet structure that was spatially diffuse across the coastline and displaced more shoreward. Simulations in which coastal valleys were removed and replaced by a larger coastal mountain range resulted in little modification to the coastal jet intensity or offshore displacement. ¿ American Geophysical Union 1996