Thursday brought more frustrating news for airlines and their customers with more than 2,100 U.S. flight cancellations as carriers continued efforts to restore flight networks hobbled by bad weather and staffing shortages brought on by coronavirus infections.

Southwest Airlines, United Airlines and regional carrier SkyWest Airlines continued to be among the hardest-hit, according to data provided by FlightAware, a website focused on aviation data. The elevated number of cancellations showed no signs of easing two weeks after emerging on Christmas Eve.

Southwest had canceled 646 flights, about 21 percent of scheduled departures. United, which had shown improvement in recent days, canceled 236 flights, about 11 percent of the carrier’s scheduled departures. SkyWest canceled 264 flights, about 11 percent of flights scheduled for the day. The Utah-based carrier, which partners with United, Delta Air Lines and American Airlines to carry passengers on smaller routes, has struggled for days.

Friday already is shaping up to be another difficult day. By early evening Thursday, airlines already had canceled more than 1,600 U.S. flights.

After nearly two years of pandemic-related turmoil, airlines were hoping for a strong finish to 2021. After forgoing holiday celebrations in 2020, Americans were eager to reunite with loved ones. With travel volumes expected to reach near pre-pandemic levels, airlines scrambled to hire workers, offering bonus pay to ensure adequate staffing levels. The surge of the omicron variant around Thanksgiving upended those plans.