A powerful storm that crept across the country dropped snow, freezing rain and sleet on the Mid-Atlantic region and headed northeast Sunday, turning NFL playing fields in Pennsylvania into winter wonderlands, dumping a foot of snow in Delaware and threatening to create a messy Monday commute in the Northeast corridor.

The storm forced the cancellation of thousands of flights in the United States and slowed traffic on roads, leading to a number of accidents, including a fatal crash on the Pennsylvania Turnpike near Morgantown that caused a series of fender-benders involving 50 cars and stranded some motorists for up to seven hours. More than two dozen vehicles were involved in another series of crashes on nearby Interstate 78.

What was forecast in the Philadelphia area to be a tame storm system with about an inch of snow gradually changing over to rain mushroomed into a full-blown snowstorm that snarled midafternoon traffic along I-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, in which he was a passenger. They 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.

A foot of snow was reported in Newark, Del. Philadelphia International Airport received 8.6 inches Sunday, more than it had all of last year. Other areas received far less. A little over an inch was reported in Pennsylvania’s Lehigh Valley, which usually is hit harder than downtown Philadelphia.

The forecast for early Monday remained up in the air for the Northeast, depending on how quickly the system moved and temperatures rose, according to the National Weather Service.

The expectation was for another weather system moving out of Virginia to follow the same path as Sunday’s storm overnight. It was expected to dump icy drizzle and eventually freezing rain from Philadelphia northward through the New York City area and into Boston, National Weather Service meteorologist Greg Heavener said.

“The commute for people in the Philadelphia area and north, to northern New Jersey and New York, will likely be disrupted by freezing rain,” he said.

The tracking Web site Flight­Aware estimated that more than 2,500 flights were canceled nationwide as of Sunday evening and that more than 6,000 were delayed. That followed two days of similarly difficult travel conditions.

Philadelphia International Airport had a temporary ground stop Sunday afternoon with snow totals around four to six inches. Spokeswoman Stacey Jackson said a number of passengers were expected to remain in the airport overnight because hotels had been full for several days.

The conditions were not limited to the East Coast. North Texas started to thaw out 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.

A snowstorm that hit along the Utah-Arizona border left hundreds of travelers stranded on I-15 overnight into Sunday. The Arizona Highway Patrol said passengers in about 300 vehicles became stranded after up to 10 inches of snow and slick road conditions prompted the closure of part of the highway. No serious injuries were reported.

— Associated Press