Hurricane Joaquin strengthened in the Atlantic on Wednesday and is expected to become a major hurricane, the US National Hurricane Center (NHC) said, although not all forecast models agreed on whether it would make landfall in the United States.

The warnings came as the northeastern United States faced a pounding rainstorm that flooded streets and snarled travel from Washington to Boston.

Joaquin is the third hurricane of the 2015 Atlantic season. By Wednesday night, it has intensified into a Category 3 on a scale of 1 through 5, with maximum sustained winds of 115 miles per hour (185 kph).

The hurricane was about 80 miles (129 km) east-northeast of the central Bahamas and continues to strengthen, the NHC said.

The governors of New York and Connecticut, and emergency-management officials in New Jersey - states all hard hit by 2012's Superstorm Sandy and already facing heavy rains unrelated to Joaquin on Wednesday - warned residents to begin preparations for a possible severe storm.

"Our state has seen the damage that extreme weather can cause time and time again," said New York Governor Andrew Cuomo. "Take precautions for more heavy storms in the coming days."

Joaquin is expected to move near or over portions of the central Bahamas on Wednesday night and Thursday, the NHC said.

"Additional strengthening is forecast during the next 48 hours, and Joaquin is expected to become a major hurricane during the next 24 hours," it said.

Bahamas residents on the islands closest to Joaquin's path, which include Rum Cay, Long Island, Exuma and Eleuthera, were stocking up on food and drink supplies and boarding up homes and businesses.

The central Bahamas and northwestern Bahamas were under a hurricane warning, the NHC said.

A major hurricane is considered to be one with winds of at least 111 miles per hour (179 kph), the threshold for Category 3 of the five-step Saffir-Simpson scale, where Category 5 is the most severe.

A complicated atmospheric pattern has made Joaquin particularly difficult to track, according to Weather Channel forecasters, who said it was too soon to determine what impact Joaquin could have on the US East Coast starting this weekend.

The last hurricane to make landfall in the continental United States was Arthur, which hit North Carolina as a Category 2 storm in July 2014, bringing high winds, driving rain and storm surges up the East Coast.

In October 2012, Sandy slammed into the New York metropolitan area, killing more than 120 people and causing some $70 billion in property damage, primarily in New York and New Jersey.

White House spokesman Joshua Earnest said the Federal Emergency Management Agency was monitoring the storm and preparing for a possible hit.