Thick fog could complicate the morning commute in Orange County on Monday, the start of a modest cooling trend that should last through Thursday, the National Weather Service says.

Forecasters issued a dense-fog advisory through 9 a.m., with visibility potentially dropping to a quarter mile or less; drivers are urged to slow down, use low-beam headlights and leave plenty of room ahead.

Article Tab: Fog Saturday evening at the Huntington Beach Pier.
Fog Saturday evening at the Huntington Beach Pier.

A weak system of low-pressure air will slowly make its way inland through Thursday, thickening the marine layer and bringing a few degrees of cooling.

A weak high-pressure system should follow, pushing temperatures a bit higher for the weekend.

After the fog clears on Monday, the day looks mostly sunny, with highs of 79 to 84 degrees inland, 70 on the coast.

Tuesday also starts off with potentially dense fog, then stays partly cloudy, with highs of 80 to 85 degrees inland, 72 on the coast.

Then the forecast is the same Wednesday through Sunday: some clouds and fog in the morning, mostly sunny skies later.

Inland highs remain modest, then move a few degrees higher. Expect 79 to 84 Wednesday, 78 to 83 Thursday, 81 to 86 Friday and 82 to 87 Saturday and Sunday.

On the coast it’s 71 Wednesday and Thursday, 73 Friday, and 75 Saturday and Sunday.