Understanding the formation and evolution of offshore fog remains a research challenge due to lack of surface and upper-air observations over the ocean and ambiguity of characterizing 3D cloudiness and fog by using satellite data. In this study, we have analyzed a case of offshore fog that was wide spread over the US West Coast waters in mid-April 1999. Based on buoy and coastal station data as well as numerical simulation results, we find that offshore fog occurs under conditions of the cloud-topped marine atmospheric boundary layer that has been modified on long over-water trajectories where strong subsidence and marine inversion are present. The primary mechanism for this fog formation is cloud-top cooling that promotes cooling of the entire marine atmospheric layer in spite of a sea surface warmer than the air. Subsequent to this cooling, saturation takes place. It is important to emphasize that the cloud lowering leading to fog formation must be viewed in a Lagrangian framework in contrast to the usual cloud thickening process that leads to fog when viewed in the Eulerian framework. Although the surface fluxes apparently have a secondary role in the fog formation stage, the forcing by air-sea interaction processes significantly contribute to fog evolution and its eventual dissipation. Model sensitivity studies confirm that the formation and evolution of offshore fog occurs within the delicate balance of advected cloudy marine flow under a relatively narrow range of strong subsidence, marine inversion, and wider range of the fluxes at the air-sea interface. Change of synoptic conditions and land-induced circulation further modify the marine layer and the fate of presence or absence of offshore fog.