The commuter rail lines that serve Long Island, Westchester County and Connecticut will also be shut down.

Officials decided to go ahead with the transit shutdown, which they had first mentioned on Thursday as a possibility at a City Hall briefing, as the city was evacuating hospitals and nursing homes in low-lying areas. State officials continued arrangements for coordinating emergency services and restoring electricity if the storm does the kind of damage many fear.

Some Atlantic City casinos made plans to stop rolling the dice and turn off the slot machines by 8 p.m. Friday. The naval submarine base in Groton, Conn., sent four submarines out to ride out the storm deep in the Atlantic Ocean. And Gov. Chris Christie of New Jersey said that all lanes of 28-mile stretch of a major highway in Ocean County would go in only one direction — westward — beginning at 6 p.m. on Friday to help speed the trip away from Long Beach Island. Those preparations came as states of emergency remained in effect in New York, New Jersey and Connecticut. Homeowners scrambled to cover windows with plywood and boaters struggled to get their vessels away from docks. In New York, apartment dwellers with balconies and terraces hauled in patio furniture and potted plants, and stores ran short on staples like batteries, flashlights and bottled water. In shore towns on Long Island and in New Jersey, vacationers waited in lines at gasoline stations and watched as emergency crews piled sandbags on low-lying beach roads.

The hurricane watch for the city was a formal indication that forecasters saw a potential threat within 24 to 36 hours. It was issued 14 hours after Mayor Michael R. Bloomberg said the city was ready with "evacuation contingencies" for low-lying places like Coney Island in Brooklyn, Battery Park City in Lower Manhattan and parts of Staten Island and the Rockaways in Queens — areas that are home to 250,000 people.

The mayor said Thursday that the city was ordering nursing homes and hospitals in those areas to evacuate residents and patients beginning at 8 a.m. Friday unless they receive special permission from state and city health officials, among them the city's health commissioner, Dr. Thomas A. Farley, who, the mayor noted, was chairman of the community health sciences department at Tulane University when Hurricane Katrina hit New Orleans in 2005.

The evacuation order covered 22 hospitals, nursing homes and other facilities for older people.

The city also ordered construction work halted until 7 a.m. Monday. With the worst of the storm expected on a weekend, a time when relatively few construction crews would normally be on the job, the Buildings Department said Friday that its inspectors were checking construction sites to see that equipment had been secured. The department said it would check over the weekend that builders complied with the no-work order.

Anticipation of the hurricane disrupted other rituals of late summer. New York University postponed its move-in day for undergraduates, which had been scheduled for Sunday, James Devitt, a university spokesman, said. Students will not be able to move into university housing until Monday. Columbia University also shifted its move-in day from from Sunday to Monday and Tuesday, according to an e-mail from Brian Connolly, a university spokesman. He also said that campus events on Sunday and Monday, including a welcome reception and a convocation ceremony, had been cancelled and would be rescheduled.

At a City Hall briefing on Thursday evening, the mayor said the five hospitals in the low-lying areas were reducing their caseloads and canceling elective surgeries on Friday to be ready for emergencies over the weekend. One, Coney Island Hospital, is to begin moving patients to vacant beds in other parts of the city on Friday, he said.

Mr. Bloomberg said he would decide by Saturday morning whether to order a general evacuation of the low-lying areas.

He also said he was revoking permits for events in the city on Sunday and in the low-lying areas on Saturday. The Sunday cancellations apparently included a concert on Governors Island by the Dave Matthews Band. A statement on the band's Web site said people with tickets for that show should attend the Friday or Saturday performance. But the Web site said to check for updates on Friday.

Sydney Ember, Matt Flegenheimer and Stacey Stowe contributed reporting.