A National Weather Service dense fog advisory for the beaches and valleys will expire at 9 a.m.
"Areas of dense fog will persist past sunrise, then clear back towards the coast during the morning," according to the weather service. "Lowest visibilities in San Diego County are expected near higher coastal terrain inland to the mesas and the far western valleys."
Forecasters said the fog may cause visibility to drop to less than a quarter-mile at times on some area roadways, including portions of Interstate 5 and 15. Drivers were advised to slow down, turn on their headlights and stay back from cars ahead of them.
Foggy conditions could also lead to early morning delays at Lindbergh Field in San Diego and McClellan-Palomar Airport in Carlsbad, according to the NWS.