A thick blanket of fog descended over the Seattle area Tuesday morning, delaying flights at Sea-Tac Airport and forcing diversions of landing aircraft ahead of one of the busiest travel weekends of the year.

The National Weather Service has issued a dense fog advisory, meaning visibility will be less than a quarter of a mile, through noon.

It also cautioned of freezing fog in areas around Puget Sound, which could cause pockets of black ice on bridges and roads during the morning commute.

More than a dozen airplanes that were due to land have been sent to nearby airports, while others that had enough fuel on board are being kept in the air, according to Perry Cooper, a spokesman for the Port of Seattle.

Text “update” to 206-347-8045 or use the Sea-Tac Airport app to review wait times and find your flight and terminal.

The Port of Seattle suggests travelers use light rail and other alternatives to avoid the traffic and congestion of the arrival and departure roadways.