Airlines planned to resume flights in and out of New York and Boston airports Monday after they were shut down over the weekend by Tropical Storm Irene.
Nearly 12,000 flights were canceled nationwide by Sunday afternoon, according to a flight-tracking service.
Airports remained closed in around New York, the nation's busiest region for air traffic, and airlines canceled their entire schedule of flights to New York and Boston for the day.
Even as airports reopened further south, including in Washington, Philadelphia and Richmond, Va., United, Continental, Delta, American, JetBlue, Southwest, AirTran and US Airways canceled hundreds of Monday-morning flights to New York's and Boston's airports.
Airlines declined to say how many passengers have been grounded since Irene came ashore in North Carolina on Saturday. FlightAware, which tracks cancelations, put the total around 650,000, noting that many of the more than 11,800 flights canceled so far were on regional airlines that use small planes. Some travel experts thought the number of people affected was likely much larger.
Finding open seats could be difficult this week, the last before the summer vacation season ends.
"We're coming into the Labor Day holiday weekend, so a lot of those flights are already full," said Todd Lehmacher, a spokesman for US Airways.
The longer that the New York area's Kennedy, LaGuardia and Newark, N.J., airports are shuttered, the more that travel delays will continue rippling across the country.
New York City officials said they weren't sure when mass transit would be fully restored after shutting down Saturday, though some local bus service resumed Sunday evening. The subways remained closed, and downed trees and flood waters still obstructed commuter train tracks across the region. Those disruptions also affected the airlines' plans.
"It's really dependent upon mass transit and the airport being ready to support the start-up," JetBlue CEO Dave Barger told NBC.
Bus and long-distance train companies also saw lingering effects from Irene, which was downgraded early Sunday to a tropical storm. Greyhound scrubbed bus travel between Washington and New York, and cruise lines changed some of their itineraries as the storm made its way from the Bahamas to Boston.
Amtrak said trains from New York to Florida will be canceled Monday, as will the car train between Lorton, Va., and Sanford, Fla. Some lines in North Carolina and Florida will be open. An Amtrak spokeswoman said no decision had been made by early Sunday evening about service in the Northeast corridor, where crews were still inspecting tracks.
Airlines said passengers should call ahead and make sure they have a confirmed seat before going to the airport, but the 670 flights that FlightAware said airlines had canceled for Monday is a very small percentage of the nation's daily flights.
Airlines also moved several hundred planes out of the storm's path to avoid damage, which will further slow the return to normal service.
When blizzards hit the East Coast in December and February, it took some passengers days to get home. That could happen again.
Sara Hesselsweet of Norwalk, Conn., and her family were to fly home Sunday from vacation at Lake Tahoe on the California-Nevada border. After their flight was canceled, American Airlines told her it couldn't find seats for her, her husband and 2-year-old son until next Saturday.
So the family decided to fly from Reno, Nev., to Dallas and on to Chicago, where they would rent a car to drive back to Connecticut.
"We checked Philadelphia, D.C., Boston, the Carolinas -- we couldn't get a flight anywhere," said Hesselsweet, sitting amid a pile of carry-on bags in the Reno airport.