A powerful storm that crept across the country dumped a mix of snow, freezing rain and sleet on the Mid-Atlantic region and headed northeast Sunday, turning NFL playing fields in Pennsylvania into winter wonderlands, threatening as much as a foot of snow in Delaware and New Jersey and raising concerns about a messy morning commute.

The storm forced the cancellation of thousands of flights across the U.S. and slowed traffic on roads, leading to a number of accidents, including a fatal crash on the Pennsylvania Turnpike near Morgantown that led to a series of fender-benders involving 50 cars.

What was forecast in the Philadelphia area to be a tame storm system with about an inch of snow followed by rain mushroomed into a full-blown snowstorm that snarled traffic along Interstate 95 in Pennsylvania from the Delaware to New Jersey state lines.

Paul Jones, 24, a youth hockey coach from Warminster in the Philadelphia suburbs, was on his way to a game in Lancaster when he got stuck — along with his fiancee, another coach and three players — in a major backup on the turnpike.

The roadway was “snow-covered, slick,” Jones said in an interview from the car, where he was a passenger and had been at a standstill for more than an hour.

“People are in and out” of their vehicles, he said. “Kids are having a snowball fight on the side of the road, making snow angels, people are walking their dogs.”

The National Weather Service said the low pressure system from North Carolina north to New England was being fed by disturbances from the southwest and moist air off the Atlantic.

The forecast called for the wintry mix to continue through Sunday, turning to rain early today. Total snow accumulation in some sections of southeastern Pennsylvania, Delaware and southern New Jersey could reach 9 to 11 inches, while other areas could see as little as an inch or 2, said Valerie Meola, meteorologist with the National Weather Service in Mount Holly, N.J.

The snow fell so heavily in Philadelphia on Sunday that yard markers at Lincoln Financial Field — where the Eagles beat the Detroit Lions — were completely obscured. It was almost as bad in Pittsburgh, where the snow intensified after opening kickoff.

Virginia, parts of West Virginia and the metropolitan Washington, D.C., area braced for blackouts under steady freezing rain, wet snow and sleet. Parts of northwest and southwest Virginia and southern West Virginia were getting snow, while sleet and freezing rain prevailed west and north of Richmond.

In Maryland, a chain-reaction accident on Interstate 81 in Washington County involving more than 20 vehicles delayed snow removal efforts for hours. The highway was closed for more than three hours after a tractor-trailer ran into the median to avoid cars that had spun out. It was hit by another tractor-trailer that overturned and spilled its load. Several other tractor-trailers ran off the road and jackknifed as their drivers tried to avoid the crash.

North Texas started to thaw out slightly Sunday after two days of a near standstill with icy roads and low temperatures. About 400 departures from Dallas-Fort Worth International Airport were canceled Sunday. It will likely be a couple of days before the ice that’s coated the region melts completely.