Chicago-area drivers are urged to be extra careful Friday as dense fog is expected to cause low visibility and dangerous road conditions.

The National Weather Service issued a dense fog advisory that will remain in place until 4 p.m. Friday in northeast Illinois and northwest Indiana. Similar advisories have been in place for much of this week across the Chicago area.

“While it might be uncommon, it’s not record-breaking or unheard of to have several days of dense fog,” weather service meteorologist Casey Sullivan told the Chicago Sun-Times on Thursday.

The fog comes from a warmer patch of air moving over a cold ground, but because the air isn’t warm enough to melt the snow, it’s taking longer for the fog to clear out and visibility to improve.

Visibility can be reduced to a quarter-mile or less with this type of fog, according to the weather service. Drivers should slow down, use headlights, leave plenty of distance between themselves and other cars and be careful of slick patches on the roads.

Rain expected Friday afternoon could clear up the fog in some areas.

More fog is expected Saturday. By Sunday, the forecast is cloudy but not foggy, according to the weather service.