Dense fog continues to cover the Chicago area, resulting in canceled and delayed Southwest flights out of Midway Airport on Christmas Eve. Conditions are expected to improve throughout the day.

The fog started impacting Southwest flights Saturday night and into Sunday morning out of Midway, according to a statement from Southwest.

As a result, Southwest canceled 58 flights for Sunday, most of which were departing from Midway.

“We continue to experience some delays and diversions as fog remains a factor,” the statement reads. “We apologize for the inconvenience to our customers as we work to get them to their destination safely.”

Midway is the airline’s fourth busiest airport operation and over 200 departures a day are scheduled over the holiday weekend, according to Southwest’s statement.

Sunday’s cancellations come almost exactly a year after the airline’s operating system malfunctioned and led to thousands of canceled and delayed flights. Southwest reached a $140 million settlement last week because of the meltdown.

Over the last 24 hours, 219 arriving and departing flights have been delayed at Midway and 49 have been canceled, according to the Chicago Department of Aviation. O’Hare International Airport has experienced 191 delayed flights, and 16 have been canceled.

It’s the second day in a row the National Weather Service has issued an advisory for fog in the Chicago area. Portions of central, east central, north central and northeast Illinois and northwest Indiana are impacted.

The dense fog, impacting visibility within a quarter mile or less, makes for hazardous driving conditions, the weather service said.

After a foggy start to the day, the weather will turn mostly cloudy with highs in the 50s, the weather service said. The low Sunday evening is expected to be in the high 40s.

Christmas Day will be rainy, with highs in the mid-50s, according to the weather service. The rain will continue into Monday night, with the low around 45 degrees.