The advisory is in effect until 9 a.m. for the San Francisco Bay Area.
Visibility has been recorded at less than a quarter-mile, with near-zero visibility at times, according to the weather service.
Drivers are advised to slow down, use headlights and leave plenty of distance between cars, weather officials said.
The California Highway Patrol has also warned about foggy conditions on Bay Area bridges and highways.
Advisories from earlier this morning remain in effect for the Bay, Carquinez and Benicia-Martinez bridges. An alert has been issued for the Dumbarton Bridge, as well.
A fog advisory for San Mateo County was issued at 1:11 a.m., with a heavy fog alert in place at 1:08 a.m. for a section of northbound Interstate Highway 280 around Edgewood Road near Redwood City.
Alameda and Santa Clara counties are also facing soupy roads this morning. Fog advisories were in effect just after 5 a.m. and before 2 a.m. for those counties, respectively.
State Highway 84 in both directions at the Dumbarton Bridge near Newark and northbound U.S. Highway 101 at Ellis Street near Mountain View were also issued heavy fog warnings this morning, according to the CHP.