A huge dome of high pressure at ≈ 18,000 feet has toppled many record temps in Northern California in the last few days. All this heat has caused low-pressure to expand over the East Bay making for unreliable weak winds for the Pt. Isabel, Berkeley and Sherman Island.
As that dome of high pressure aloft weakens the surface low-pressure moves back towards the Central Valley but as is often the case it leaves a lobe of low-pressure just north of the Bay Area. This sets up a South to North pressure gradient which along with southerly winds just aloft creates a Marine Surge of cool foggy air which starts near Southern California and ultimately reaches the Golden Gate.
Forecasting the timing of such an event is very tricky since it hinges on the time the upper high-
pressure shrinks and the surface low-pressure retreats as well as the time it takes the Marine Surge to enter the Bay Area
To make things more complex the marine layer pauses in its northward movement every time it encounters a gap in the Coast Range or a valley like the Salinas Valley.
This animation shows the progress of the Marine Surge this morning as well as some of the variables that determine its flow.
Here is a time-lapse video of a Marine Surge as seen from the Berkeley hills.