Atlanta CNN  — 

With much of the United States still reeling from a nearly weeklong winter storm, flight cancellations and delays and other transportation challenges continued to pile up on Christmas Day.

At least 3,181 flights within, into or out of the United States were canceled Sunday, according to flight tracking website FlightAware. And delays of flights still able to takeoff numbered roughly 6,800. Christmas Day is traditionally a light day for passenger flights.

Demonstrating the sheer size and widespread effects of the storm, it was an airport in the Deep South and another out West that were most affected Christmas Day. Hartsfield-Jackson Atlanta International (ATL) – the world’s busiest airport for passengers – saw the second most cancellation and delays Sunday.

No. 1 was more than 1,000 miles away out in the Rocky Mountains with Denver International. And even farther out West, Harry Reid International (LAS) in Las Vegas had the third-most cancellations.

Denver International was keeping a sense of humor during the stressful event, posting a tweet of what appeared to be an airport worker playing “Jingle Bells” on a fiddle.

The storm’s effects in parts of the West are abating, though. The temperature at 8 p.m. MT at Denver International was still above freezing at 38°F (3°C).

In hard-hit western New York, things were still too rough for humor.

Buffalo Niagara International Airport (BUF) tweeted that it would remain closed entirely until 11 am. Tuesday, December 27 – extending out a closing originally set to end at 11 a.m. Monday.

The temperature at BUF at 10 p.m. ET was 20°F (-7°C) with wind speeds of 24 mph, according to the National Weather Service.

A rough week for flying

David Zalubowski/AP
A pair of travelers sleep while others line up to pass through a security checkpoint in Denver International Airport on Friday.

The massive storm’s arrival was ill-timed for travelers who had started pushing Christmas week flying numbers back toward pre-pandemic levels.

On Christmas Eve, there were a total of 3,487 flights canceled, according to FlightAware. Friday was the worst day with 5,934 cancellations, while Thursday saw almost 2,700 cancellations.

This megablast of winter weather across the eastern two-thirds of the nation is forecast to slowly moderate into the last week of the year. As of 5 a.m. ET Monday, there were more than 1,300 preemptive cancellations within, into or out of the US.

Bus and train service

On Sunday morning, Greyhound posted its latest bus service disruptions, which were centered around routes into and out of Buffalo and Syracuse in New York.

If you’re taking a Greyhound bus, you can click here for the latest information for your bus.

Amtrak’s last service alert was posted Friday. It said then the severe weather “has caused Amtrak to make several service and cancellations from now through Sunday, December 25.”

The train company said impacted customers should be able to take trains leaving at similar times or on another day. You can click here to get the latest Amtrak service alerts on delays or to @AmtrakAlerts on Twitter. Go to @AmtrakNECAlerts for Northeast alerts.

CNN’s Danielle Wiener-Bronner and Rebekah Riess contributed to this report.