• FedEx and UPS warned customers that holiday deliveries could arrive late because of bad winter weather.
• Snow, sleet, high winds and bitter cold from Winter Storm Elliott have swept through much of the U.S.
• More than 6,000 U.S. flights have been canceled and more than 20,000 delayed this week.

FedEx and United Parcel Service warned that packages could arrive late this week as a massive winter storm brought high winds, bitter cold and snow to large swaths of the United States ahead of Christmas weekend.

Severe weather was already snarling air travel during what is expected to be one of the busiest travel periods since before the pandemic.

"FedEx Express experienced substantial disruptions at our Memphis and Indianapolis hubs last night due to severe winter weather that has been moving across the United States," FedEx said Friday. It said packages set for delivery on Friday and Saturday, which is Christmas Eve, could be delayed across the country.

UPS said severe weather "across several regions of the U.S. are impacting the UPS Air and Ground network, including UPS hubs in Louisville, Kentucky and Rockford, Illinois. As a result, some delivery and pickup services in these areas will be affected."

The warnings come during one of the busiest times for package delivery, ahead of Christmas Day on Sunday.

The massive winter storm made getting home for the holidays a challenge for thousands of travelers. Airlines cancelled more than 7,000 flights and delayed more than 20,000 from Wednesday through Friday afternoon, according to flight-tracker FlightAware. The period includes some of what airlines expect to be the busiest days of the holiday period. Snow and sleet in the Pacific Northwest also disrupted flights.

Federal forecasters warned about treacherous road conditions, dangerously low temperatures and high winds in cities from Chicago to Boston. The National Weather Service had parts of Florida, including Tampa and Orlando, under a freeze warning Saturday morning.

On Thursday, 10% of U.S. airlines' scheduled flights were cancelled while almost half were delayed, arriving late by an average of around 70 minutes, FlightAware data showed.

More than to 4,800 U.S. flights were cancelled on Friday.

Southwest Airlines canceled more than 900 Friday flights, about a fifth of its operation, while nearly 1,400, a third of its schedule, were delayed, according to FlightAware. Nearly 400 of Seattle-based Alaska Airlines flights were canceled, close to half of its operation.

Alaska warned travelers on Friday that it could take days "multiple days" to rebook travelers because flights are so full during the holidays.

"Our contact centers are experiencing long hold times as they try to help thousands of guests, and we're working around the clock to reunite guests with their bags," it said in an update. "We strongly encourage guests to reassess their travel plans due to limited availability."

Airlines aim to cancel flights as far in advance as possible so travelers, crews and planes aren't stranded at the airport during bad weather.

American, Delta, United, Southwest, JetBlue, Alaska, Spirit and other carriers waived change fees and fare differences for more than 50 airports if travelers can fly later.

Airlines had hoped for a repeat of the relatively smooth Thanksgiving travel period to end what has been a rocky year for carriers, crews and customers alike due to bad weather and labor shortages.

Carriers are likely to update investors on the financial impact of the storm when they release quarterly results in January, or possibly earlier.