Joaquin strengthened into an extremely dangerous Category Four hurricane Thursday as it barreled through the Bahamas, forecasters said, while the US East Coast prepared for a weekend of heavy rainfall.

The storm was packing maximum sustained winds of 130 miles (210 kilometers) per hour and could grow even stronger over the next 24 hours, the National Hurricane Center said.

It will move near or over portions of the central Bahamas Thursday and into the night, dumping heavy rain, and pass near or over the northwestern Bahamas on Friday, the NHC said.

"Joaquin becomes an extremely dangerous category four hurricane," the latest NHC bulletin said.

The storm has potential for life-threatening damage and a course that could dump drenching rain along the US East Coast, forecasters said.

Emergency preparations began as far north as New York, and officials in the Bahamas urged people to prepare for up to 15 inches (37 centimeters) of rain.

President Barack Obama was briefed on the preparations, the White House said.

Classes were called off in the Bahamas, some flights were cancelled and cruise ships headed for the popular vacation destination were diverted elsewhere.

As the storm is moving at a relatively slow speed, "a catastrophic situation may unfold there with a prolonged period of intense hurricane conditions," according to www.weather.com.

After hitting the central Bahamas Thursday, Joaquin will move up into the northwestern Bahamas, shifting to a course that could take it up the east coast of the United States, the NHC said.

Torrential rains are possible in the Carolinas and Virginia, US media reported.

As of 1800 GMT, the eye of Joaquin was 70 miles south of the island of San Salvador, the NHC said.

A dangerous and life-threatening storm surge will raise water levels by as much as 10 feet above normal tide levels in parts of the central Bahamas, it said.

The hurricane reached Category Three status late Wednesday, making it a major hurricane, and then Category Four on a scale that maxes out at five.

The Bahamas are an island chain that at its most northern point sits off the coast of southern Florida.

- Not the first -

In Virginia, which was already hit by flash flooding Tuesday, the governor declared a state of emergency to prepare for more heavy rain even before Joaquin arrives, if it does. Authorities in New Jersey did the same.
"The forecast of up to 10 inches of rain in areas across Virginia could result in floods, power outages and a serious threat to life and property," Governor Terry McAuliffe said.

In Virginia, the Norfolk Naval Station, the biggest US naval base, was put on notice to be ready for the possibility of destructive winds.

Local NBC affiliate WAVY reported that ships were told to be prepared to get underway within 48 hours if necessary.

Joaquin is the third hurricane of the 2015 Atlantic season, which began in June and ends in November. Peak activity usually occurs in September.

But the most destructive weather pattern so far this year was Tropical Storm Erika, which killed around 30 people and caused extensive damage in August on the small Caribbean island of Dominica.

The National Oceanographic and Atmospheric Administration had predicted a less active hurricane season than usual because of the El Nino phenomenon, which inhibits storm formation.

Forecasters earlier this year also noted the dampening effects of below-average sea-surface temperatures in the tropical Atlantic. Hurricane formation is associated with warmer waters.