(CNN) – Thousands of US flights were canceled on New Year’s Day because a combination of COVID-19 and winter weather slowed travel. Flights have been canceled direction Steady since Christmas Eve, it reached a new peak on Saturday morning as millions travel during the holidays.

Data from the FlightAware website shows that more than 4,200 flights have been canceled worldwide as of Saturday noon, or about 10% of UTC.

Airlines are already grappling with the omicron variant, which has led to an unprecedented rise in virus cases. Many airline employees were unable to work. The Federal Aviation Administration has also warned that more of its employees are testing positive, which could restrict flights.

Now it adds a new challenge to the traveler’s problems: a major storm hits the Rocky Mountains and the Midwest, bringing ice and heavy snow. Chicago in particular has been hardest hit. Airlines have canceled more than half of flights from Midway International Airport and more than 40% from O’Hare International Airport. Nearly a third of flights at Kansas City International Airport have been cancelled. In Detroit, airlines canceled one in five flights.

Delta Air Lines told CNN Business that it expects to cancel 200 to 300 flights from more than 4,000 daily flights over the weekend. He also recommended that people traveling to Chicago, Detroit, Salt Lake City, Seattle, and the central and southern Rocky Mountain regions consider changing their travel plans due to weather conditions.

Southwest Airlines told CNN Business that all of its problems were caused by time.

More than 11,000 flights have been canceled since Christmas Eve, according to data from FlightAware.

Chris Isidore contributed to this story.