Forecasters said on Friday that there was a 90 percent certainty that Hurricane Sandy would make landfall on the East Coast, but cautioned that it was too early to say where the giant storm would strike or how intense its winds would be when it hit.
The storm, now moving northwest at about 10 miles per hour, may stay off the coast in the Atlantic Ocean until Monday or Tuesday, but will likely combine with a colder weather system from the west to dump more than one foot of snow — perhaps as much as two feet — in the Appalachian Mountains of West Virginia, and cause strong winds all the way to the Ohio River Valley and eastern Great Lakes region.
The storm is also expected to dump as much as 10 inches of rain in the area where it makes landfall and to create a significant storm surge that will lead to flooding throughout a large coastal area, perhaps most seriously in Delaware, forecasters said.
“We expect a long-lasting event — two to three days for most people,” said James Franklin, branch chief of the National Hurricane Center in a conference call on Friday. It is, Mr. Franklin said, “a very large system.”
On Friday morning, the hurricane tore through the Bahamas with 100-m.p.h. winds, after killing at least 28 people in the Caribbean. By noon, the system was moving north out of the Bahamas, as a Category 1 storm, with wind speeds of 80 m.p.h., Mr. Franklin said.
The hurricane is likely to turn northeast late Friday or Saturday, roughly parallel to the Carolina coast, forecasters said.
Coastal areas of the United States, from Florida to North Carolina, were under a tropical storm watch Friday.
Meteorologists say that while early storm projections can be unreliable, this storm will cause major disruptions in an area larger than Hurricane Irene in 2011, which caused billions of dollars of damage.
“It really could be an extremely significant, historic storm,” said Brian McNoldy, a senior research associate at the University of Miami, explaining that conditions are similar to those that created the famous “perfect storm” of 1991.
The chain of events that would make Hurricane Sandy develop into a grave threat to the coast involves a storm system known as a midlatitude trough that is moving across the country from the west. If the systems meet up, as many computer models predict, the storm over land could draw the hurricane in.
“Now you’ve got this giant storm complex with a lot of energy,” said Dennis Feltgen, a meteorologist and spokesman for the National Hurricane Center.
In New York, officials were preparing for a strong possibility of transit closings, with winds expected to be above the range that would prompt the Metropolitan Transportation Authority to halt some service.
The National Weather Service is predicting sustained winds of 40 to 50 miles an hour starting late Monday for the New York region. The transportation authority’s hurricane plan calls for the “orderly shutdown of service before the arrival of sustained winds of 39 mph or higher” in the elevated portions of the subway system and the agency’s railroads. Most of the city’s subway lines contain some outdoor or elevated stretches.
The authority did not rule out the possibility of shutting down the entire subway system, as it did, for the first time, in advance of Tropical Storm Irene last year.
Around the city, high school admissions tests scheduled for Sunday were postponed, Mayor Michael R. Bloomberg urged New Yorkers to stay out of city parks starting on Sunday, and the Buildings Department ordered all outdoor work at construction sites to halt Saturday evening.
In New Jersey, mandatory evacuations are in place for Sunday for several coastal areas, including barrier coastal islands and Delaware Bay communities in Cape May County and in Brigantine. Voluntary evacuations for those areas were being implemented for Friday and Saturday.
Such storm combinations have happened before: one that occurred 21 years ago developed into what is now known as the “perfect storm” off the coast of New England. That disaster was memorialized in a 1997 book and a 2000 movie by the same name.
“Essentially, all of the major models are now showing some form of phasing event similar to the perfect storm occurring sometime next week,” said William A. Komaromi, a graduate student at the University of Miami who posted an essay online on Thursday comparing Hurricane Sandy to the 1991 event.
Mr. Feltgen said that “everybody along the U.S. East Coast needs to be paying attention to this right now,” though he added that “we’re not telling people to rush to the grocery stores.” It is a time, he suggested, for watchful waiting. “Let’s not go overboard with this thing,” he said, “but you should at least start becoming aware of it.”
Mr. McNoldy said, “You want to prepare for the worst and hope for the best.” Even if the storm’s wind power weakens substantially, as Hurricane Irene showed, “rainfall can still be extremely high.” Mr. McNoldy’s posts on Twitter about Hurricane Sandy now bear the label “frankenstorm.”
The storm is approaching in the middle of preparations for the presidential election on Nov. 6 and could disrupt plans for early voting in some areas, with unpredictable results. Mark McKinnon, a former media strategist for President George W. Bush who went on to found No Labels, a group promoting bipartisanship, said that the hurricane brought to the campaigns something they both dread: uncertainty.
“Campaigns are all about control,” he said. “So in the closing days, they fear any external events that could disrupt the game plan. Ain’t no leashes for Mother Nature.”
The hurricane has killed at least 11 people in Cuba and damaged thousands of buildings there, according to The Associated Press. At least 16 people have also died in Haiti, and one person in Jamaica.
David Chen and Matt Flegenheimer contributed reporting.