Drivers heading back to work after the holiday weekend were greeted by dense fog that slowed traffic to a crawl in some parts of the Bay Area.

The fog reduced visibility to 20 feet in some parts of the Bay Area, according to the National Weather Service. The dense fog warning issued Monday morning was expected to last until 9 a.m.

"There is a lot of low-level moisture, and you combine that with clear skies and cold air, and you have your fog," forecaster Steve Anderson of the National Weather Service said. "As the air begins to warm, it will begin to clear."

The fog is also expected to blanket Tuesday morning's commute, Anderson said, and is arriving in advance of a storm that's expected to dump heavy rain and bring high winds beginning sometime Wednesday, Anderson said.

In this file photo, a thick fog blankets the trees along Park Avenue near Cabrillo Street in Capitola.
In this file photo, a thick fog blankets the trees along Park Avenue near Cabrillo Street in Capitola. (Shmuel Thaler/Sentinel)

Flights at San Francisco International Airport were expected to be delayed by more than an hour throughout the day, and travelers were advised to check their flight status. Flights at Oakland International Airport were running on time Monday morning.