Winter storms pummeling vast areas of the US are prompting widespread flight cancellations and delays from East Coast hubs to the Pacific Northwest, upending airline operations during the busy holiday travel season.

About 3,900 US flights were canceled on Friday as of 11:40 a.m. Eastern, bringing the total to more than 6,500 for the past two days, according to tracking site FlightAware.

Airports in Chicago, Cleveland, Detroit, Buffalo and Seattle were taking the brunt of the impacts with airlines scrapping between one-third and two-thirds of operations, FlightAware reported. New York’s LaGuardia was also hit hard, dropping about 30% of its normal flights. Nearby John F. Kennedy International and Newark Liberty International airports reported less than 10% cancellations, but the rate was still higher than normal.

“It’s so unfortunate,” Helane Becker, a Cowen Inc. analyst, said in an email. “It will be a mess and people will be disappointed. Delays are better than cancellations, but there is nothing they can do about the weather.”

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Airlines have for days been proactively trimming their schedules and urging passengers to reschedule flights in an attempt to make the process more orderly. That way, travelers can stay home until their new flight instead of “going to the airport and hoping for the best,” Becker said.

The Federal Aviation Administration said in a statement Friday that severe weather “moving across the Great Lakes into the Northeast will have a major impact on the national airspace system today.” In addition to problems from the storms, the agency warned of heavier-than-normal volumes in Puerto Rico, Gulf of Mexico destinations and ski country areas.

The weather woes add additional stress to airlines, some of which have in recent weeks reported slack demand in December. Carriers have been counting on a steady recovery in air travel to help them return to pre-pandemic sales levels.

Southwest Airlines Co. reported the biggest impacts among carriers Friday, canceling 798, or 19%, of its flights. Chicago’s Midway, one of its busiest airports, reported about half of its arrivals and departures were canceled, according to FlightAware.

Alaska Air Group Inc. dropped 312 flights from its schedule Friday, or 40%, as snow and ice hit its Seattle hub.