Hurricane Sandy bore down on the Eastern Seaboard's largest cities today, forcing the shutdown of mass transit, schools and financial markets, sending coastal residents fleeing, and threatening a dangerous mix of high winds, soaking rain and a surging wall of water up to 11 feet tall. Sandy strengthened before dawn and stayed on a predicted path toward Washington, Baltimore, Philadelphia and New York — putting it on a collision course with two other weather systems that would create a superstorm with the potential for havoc over 800 miles from the East Coast to the Great Lakes. About 2 to 3 feet of snow were even forecast for mountainous parts of West Virginia. The tempest could endanger up to 50 million people for days. Many workers planned to stay home Monday as subways, buses and trains shut down across the region under the threat of flooding that could inundate tracks and tunnels. Airports also closed, and authorities warned that the time for evacuation was running out or already past. Utilities brought in extra crews, anticipating widespread power failures. The center of the storm was positioned to come ashore Monday night in New Jersey, meaning the worst of the surge could be in the northern part of that state and in New York City and on Long Island. Higher tides brought by a full moon compounded the threat to the metropolitan area of about 20 million people.