The spring transition between winter and summer conditions on the northern California continental shelf occurred suddenly, over a period of days, in late March in 1981 and in mid-April in 1982. The spring transition marks the onset of the upwelling season over the continental shelf and is a response to an atmospheric spring transition associated with a sudden seasonal change in the large-scale atmospheric pressure field over the North Pacific. The 1981 and 1982 spring transitions over the northern California shelf are characterized by a drop in the water temperature over the shelf of 2¿--4¿C and a reduction of the temperature difference across the water column at midshelf from 2¿C to 0.5¿C. Isotherms and isopycnals, which were level prior to the spring transition, slope upward toward the coast after the spring transition. The mass balance for the 1982 spring transition event is roughly two dimensional, with offshore flow near the surface balanced by weaker subsurface onshore flow. The alongshore momentum balance during the 1982 spring transition is not two dimensional and includes the establishment of an alongshore pressure gradient which opposes the wind and apparently drives a poleward flow observed during a period of weak wind following the spring transition. The cross-shelf momentum balance is geostrophic, with density fluctuations contributing to the cross-shelf pressure gradient. The heat budget for the spring transition event is roughly two dimensional, with the drop in temperature due primarily to a cross-shelf exchange of heat associated with the cross-shelf circulation. ¿American Geophysical Union 1987