An East Coast blizzard that has forced nearly 7,000 flight cancelations will leave many travelers stranded through the end of the week. Runways reopened Monday evening at several major airports in the Northeast. But canceled flights into and out of Philadelphia, New York and Boston left hundreds of thousands of people scrambling for a way home. The storm and its aftermath could end up costing the airlines $100 million, one analyst predicted.

The challenge for the airlines goes beyond weather. Flights are usually full this time of year, making it difficult to rebook travelers affected by a cancellation. Seats are even more scarce than in past years because the airline industry has reduced the number of flights and grounded planes to save money and drive up prices.

"This is a bad time for a blizzard to hit the East Coast," said airline consultant Darryl Jenkins. He said it will be difficult for the airlines to accommodate all the stranded travelers in the New York area quickly enough, and some may abandon their travel plans.

The paralyzing storm in the Northeast comes a week after several inches of snow shut down London's Heathrow Airport and left travelers sleeping on terminal floors. It took five days for Europe's busiest hub airport to resume normal operations.

By afternoon, major U.S. airlines had announced more than 3,100 canceled flights for Monday. Continental, whose hub in Newark, N.J., was shut down by the storm, scrubbed 800 flights and Delta dropped 700. US Airways canceled about 830 flights.