This morning the Marine Layer is being pushed over the foothills. Looks like fall is starting early this year. BTW, a marine layer is an air mass which develops over the surface of a large body of water such as the ocean or large lake in the presence of a temperature inversion.
In the case of coastal California, the offshore marine layer is typically propelled inland by a pressure gradient which develops as a result of intense heating inland, blanketing coastal communities in cooler air which, if saturated, also contains fog. An approaching frontal system can also drive the marine layer onshore.