Hurricane Earl gained strength late Wednesday and reclaimed its status as a Category 4 storm as it roared toward the mid-Atlantic coast, where it is expected to make landfall late Thursday.

A public advisory issued by the U.S. National Hurricane Center at 11 p.m. ET said Earl's maximum sustained winds have increased to 220 kilometres per hour. The storm is large and growing, the advisory said -- hurricane-force winds extend 150 km out from its centre.

At the time of the advisory, Earl was located about 840 kilometres south-southeast of Cape Hatteras, N.C., and was travelling north-northwest at a speed of 30 km/h.

The centre has issued numerous warnings and watches, including:

• A hurricane warning from Bogue Inlet, N.C. to the North Carolina-Virginia border.
• A hurricane watch from north of the North Carolina-Virginia border to Cape Henlopen, Delaware, and from Woods Hole to Sagamore Beach in Massachusetts.
• A tropical storm warning from Cape Fear to west of Bogue Inlet, N.C., and from the N.C.-Virginia border to Sandy Hook, New Jersey.
• A tropical storm watch from Sandy Hook, N.J. to Woods Hole, Mass., and north of Sagamore Beach to the mouth of the Merrimack River, Mass.

Prior to Earl's upgrade, North Carolina Gov. Beverly Perdue declared a state of emergency Wednesday afternoon.

Officials in North Carolina's Dare County also issued mandatory evacuation orders for residents of and visitors to seven communities on the Outer Banks' Hatteras Island. Those who choose to stay are being warned that they may go for as long as 72 hours without help after the hurricane passes.

Meanwhile, Maryland Gov. Martin O'Malley also declared a state of emergency, as did Virginia Gov. Bob McDonnell.

Officials at the hurricane centre say that should Earl make a turn northeast, it could make landfall on the eastern-most tip of North Carolina late Thursday. By early Friday, tropical storm-force winds could be felt from Virginia to New Jersey.

The hurricane centre warned Wednesday that powerful swells driven by Earl will begin to arrive along the U.S. coast in the coming days.

The U.S. centre said a "dangerous storm surge will raise water levels by as much as three to five feet above ground level within the hurricane warning area. Near the coast, the surge will be accompanied by large and destructive waves."

Nova Scotians warned

Earl, or its remnants, is expected to hit the Canadian coast on Saturday morning, most likely near Yarmouth, Nova Scotia.

During a news conference Wednesday afternoon, Environment Canada officials said it is even more likely than previously thought that Earl will hit the province, with winds gusting as high as 140 km/h.

Chris Fogarty, program supervisor at the Canadian Hurricane Centre, said there is a 50 per cent chance the storm will reach Canada as a Category 1 hurricane, up from the 30 to 40 per cent chance predicted on Tuesday.

According to Fogarty, Earl is expected to arrive around 9 a.m. Saturday. However, it may arrive as much as twelve hours earlier or later, and may veer off its current course to land anywhere from Maine to Cape Breton.

Fogarty also said the tropical air mass that is currently over the Maritimes and above-average ocean temperatures may give Earl strength as he churns toward the region.

"The persistently warm water through here could be a factor (and) we could be in the upper end of a Category 1," Fogarty said.

"It has the potential to retain hurricane status longer. That's why we're wanting people to keep their heads up. It's definitely a storm with potential to be very problematic."

In a statement issued earlier Wednesday, the hurricane centre said it's still too early to issue warnings or make predictions about possible damage when the hurricane, or its remnants, hits the Maritimes.

The centre would only say "the public should certainly pay close attention regarding weekend plans that may be sensitive to wind and rain conditions."

Forecasters are calling for a 75 per cent chance of at least gale-force winds between 60 and 70 km/h in Nova Scotia. Fogarty said the storm is expected to cause power outages and flooding in some areas.

Mike Myette, emergency services co-ordinator for Nova Scotia, warned residents to prepare for the worst and have an emergency kit ready.

"This kit should include food, water, blankets, flashlights, a radio, first-aid supplies and copies of important papers," Myette said at a news conference.

"Nova Scotians need to be prepared to be self-sufficient for at least 72 hours."

He also warned residents to stay away from the water due to strong, dangerous currents, and said people should fill their cars with gas, stock up on propane and have cash on-hand.