More Fog Expected--but You May Have Mist It : Weather: Forecasters say it will be heavy, but visibility should start improving by dawn. LAX was forced to divert 20 planes after being socked in late Thursday.

Weather forecasters predict a return early this morning of the heavy fog that enveloped much of the Southern California coast late Thursday and early Friday, preventing planes from landing at Los Angeles International Airport for more than 15 hours.

But this morning’s fog shouldn’t be quite as dense or as long-lasting as the blanket that reduced visibility at LAX to 800 feet by nightfall Thursday, forcing the diversion of more than 20 inbound flights to airports in Ontario, Burbank and even Las Vegas.

Some of the passengers waited for skies to clear late Friday morning before continuing to LAX by air. Others decided not to wait and took overnight buses.

Steve Burback, a meteorologist with WeatherData Inc., which supplies information for The Times, said the fog was brought on by a gradual breakdown of a persistent inland high-pressure system that had kept Southern California unusually dry for more than two weeks.

Santa Ana winds from inland deserts gave way to gentle breezes from the Pacific, and that allowed a little moisture to sneak back in from the sea.

Burback said that by dawn today, the fog should start giving way to a low overcast that will increase with the approach of a low-pressure weather system from the north. The weather front should pass through the Southland sometime tonight, bringing a slight chance of showers in the Los Angeles Basin.

Gusting breezes from the northwest should leave skies clear by midday Sunday, with slightly higher temperatures.

Highs today in the Los Angeles area should be mostly in the 60s, after overnight lows from the mid-40s to the lower 50s. Tonight’s lows should be much the same, with high readings Sunday in the mid-60s and low 70s.

Burback said there is still no prospect of any substantial rain in the near future.

He said the high altitude jet stream that circles the Northern Hemisphere from west to east is still passing well to the north of us, and the storm systems it carries are largely bypassing Southern California.