New York
CNN
 — 

As frigid temperatures blast much of the United States and threaten more storms in the coming days, airports across the country are experiencing thousands of delays and cancellations for the third consecutive day.

As of Sunday afternoon, more than 4,000 flights within, into or out of the country, have been postponed, and more than 1,000 canceled, according to tracking site FlightAware. While an improvement from Saturday’s nearly 8,000 delays and 1,400 cancellations, the nightmarish travel weekend has seen more than 11,000 total flight delays. There were approximately 9,000 delays and 2,300 cancellations Friday.

The most affected airports continue to be those across the northern United States. Denver, which is under a winter storm warning until Monday evening, saw 10% of its airport’s incoming flights canceled today. In Chicago, about a fifth of flights going in and out of O’Hare International Airport are delayed. More than 80% of flights out of Buffalo Niagara International Airport were canceled, as Buffalo, New York, and its surrounding areas brace for several feet of snow, according to the National Weather Service.

Winter weather alerts on Sunday span from Texas to Virginia and encompass more than 45 million people, including all of Tennessee and Arkansas. Texas, notorious for its vulnerable-to-extreme-temperatures power grid, is bracing for extremely cold temperatures throughout much of the state. Ten percent of flights out of Dallas’ Love Field Airport have been canceled, as well as 16% of incoming flights.

Travel delays are exacerbating an already-difficult week for airlines and their passengers. Last week, more than 200 United and Alaska Airlines flights were canceled each day due to the federally mandated grounding of Boeing 737 Max 9 aircraft. The Federal Aviation Administration instated the order after a piece of the fuselage blew off an Alaska Airlines flight on January 5 with 177 people on board. The incident left a gaping hole in the side of the plane and ripped headrests off seats as the plane flew at 16,000 feet, shortly after taking off from Portland, Oregon.

United and Alaska Airlines, the two largest US carriers using the Max 9 aircraft, are among the airlines canceling the most flights, according to FlightAware. United was the second most affected airline behind Southwest, which doesn’t fly any Max 9 planes, with 244 cancellations. Alaska Airlines experienced the third-highest number of cancellations on Sunday, having called off nearly a quarter of its total flights.

Both airlines said last week they have canceled all flights on their Max 9 planes until mid-January.