US airlines have rolled out sweeping travel waivers as a winter storm, forecast to spread heavy snow, sleet and dangerous ice across two dozen states, threatened to disrupt air travel across a wide swath of the country heading into the weekend.

The National Weather Service has warned that the mix of freezing rain, sleet and snow could make travel dangerous and lead to power outages and tree damage in parts of the Southeast, while heavier snow is expected farther north.

The winter storm is expected to bring cold temperatures not seen since 2021, according to AccuWeather, a weather forecasting company.

Delta Air Lines cancelled flights at select airports in five states on Thursday afternoon, as carriers said the mix of ice, snow and strong winds could slow airport operations, ripple into delays and cancellations, and complicate rebooking during one of the busier winter travel periods.

“This will likely result in thousands of flight cancellations on Saturday, Sunday and Monday,” said Dan DePodwin, vice president of forecasting operations at AccuWeather.

Travel booking app Hopper estimates that upwards of 15,000 flights could be delayed due to the storm.

Southwest Airlinessaid it expects disruptions across parts of the South and East, giving customers flexibility to rebook or travel standby within a two-week window.

Delta said weather could affect flights across the Southern Plains and Southeast, including its Atlanta hub, and has cancelled flights in North Texas, Oklahoma, Arkansas, Louisiana, and Tennessee.

The carrier said it will automatically rebook customers to the next-best itinerary and offered fee-free changes for eligible customers who adjust travel within its waiver terms.

American Airlines expanded similar flexibility across a network that includes major hubs such as Dallas-Fort Worth and Charlotte, while United Airlines issued a waiver spanning much of the Mid-Atlantic and Northeast and parts of the Carolinas and Ohio Valley, including its Newark and Washington-area operations, allowing affected customers to shift travel within a set period without change fees or fare differences if they keep the same itinerary and cabin.

JetBlue issued a Northeast-focused waiver that covers New York and Boston-area airports and said it will waive change and cancellation fees for customers traveling to and from 11 cities.

Ultra-low-cost carrier Frontier Airlines said if flights are cancelled or delayed for more than three hours for domestic and international flights, respectively, travellers can request a refund.

Spirit Airlines said it is waiving modification charges and fare differences across 13 cities for travel between January 23 and 25.

Airlines urged travellers to monitor flight status closely and use mobile apps and airline websites for the fastest rebooking options as the storm advances.

Travellers have already started to prepare for travel disruptions.

Hopper said it has seen a 17% increase in travellers adding Disruption Assistance to trips for this upcoming weekend.

“Even in locations far away from the storm, such as the West Coast hubs, Denver, Chicago and Minneapolis, ripple effects of this widespread travel-halting winter storm will be felt,” said DePodwin.