Henri to leave Northeast Monday night, flood watches remain in effect

Flash flood warnings are already in effect across New York City, Staten Island and northern New Jersey due to moisture funneling into the region from an upper level system to the west of Henri.

Flooding from Henri, which continues to hold onto hurricane status with 75 mph max sustained winds, will be a widespread threat across the Northeast on Sunday.

Henri is moving north at 21 mph and is 135 miles south-southeast of Montauk Point, New York, and 175 miles south of Providence, Rhode Island. Slight strengthening is possible, and Henri is expected to be at or near hurricane strength when it reaches the coasts of southern New England and Long Island.

The track of Hurricane Henri appears to be shifting slightly more east on Saturday night.

Landfall now appears possible on far eastern Long Island or southern New England, anywhere from eastern Connecticut to Rhode Island to southeastern Massachusetts on Sunday afternoon. Earlier in the day, it appeared as though the storm might hit central Long Island.

The storm was moving quickly -- 21 mph -- and still maintaining hurricane strength.

Henri is expected to either be a weak hurricane or strong tropical storm on Sunday morning or early afternoon as it moves over or just east of Long Island and eventually into southern New England through the day on Sunday.

Rail service in parts of New York and Connecticut will be halted Sunday due to Henri.

The Long Island Rail Road will be suspending service east of Patchogue on the Montauk Branch and on the Greenport line. 

Metro-North, which serves New York and Connecticut, will be suspending service on the Wassaic Branch of the Harlem Line and on the entire New Haven Line, including the New Canaan, Danbury and Waterbury branches. 

In Connecticut, service on the rail lines Shore Line East and Hartford Line is suspended starting Sunday "until further notice."

Amtrak also announced it would be canceling all Northeast Corridor service, including Northeast Regional and Acela, between New York and Boston on Sunday.

Flight cancellations are also expected in the region.

"Port Authority expects flights to be canceled," Gov. Andrew Cuomo said during a briefing Saturday. "Obviously I would check with your airline before you go to the airport tomorrow into Monday, maybe Tuesday."

New York City Mayor Bill de Blasio defended continuing on with its "We Love NYC Homecoming Concert" Saturday evening, as the city prepares for Hurricane Henri.

"Since the real impact is not expected until well after the concert, we believe absolutely we can do both these things at once," de Blasio said during a press briefing Saturday, as the concert was set to kick off in Central Park with performers including Bruce Springsteen and Patti Smith.

The mayor urged concertgoers to go straight home at the end of the night. "If you're going to the concert ... go home right after and get ready for tomorrow," he said.

De Blasio, who declared a state of emergency for NYC earlier on Saturday, said the city is preparing for heavy rain, wind and downed trees.

-ABC News' Matt Foster