As summer turns into fall the sun has moved south of the equator. The decreased sun angle means the land and water are not as severely heated.
The high pressure system moves south with the Sun and begins to weaken. As it weakens the strong winds that caused the upwelling off the coast die down. That means the water begins to warm. The warmer water stops the generation of the summer fogs.
The reduced temperature differences between the ocean and the land curtail the daily onshore flow cycle.
As a result the air during the fall is frequently stagnant. September and October are our hottest months.
At the start of November the High can break down entirely. This sets the stage for the winter storms and the beginning of the next cycle of weather.