Hurricane Dorian is expected to suck powerful fuel from the warm waters off the Florida coast, swelling into a dangerous Category 4 storm in the coming days before it slams into the state early next week, forecasts showed on Friday.

The Miami-based National Hurricane Center (NHC) issued a hurricane watch for northwestern Bahamas on Friday, and said the risk of "devastating hurricane-force winds along the Florida east coast late this weekend and early next week continues to increase."

NHC posted an update Friday morning on the hurricane menacing the U.S. mainland.

"Dorian is likely to remain an extremely dangerous hurricane while it moves near the northwestern Bahamas and approaches the Florida peninsula through the weekend."

Here are the 5 AM AST/EDT August 30 Key Messages for Hurricane <a href="https://twitter.com/hashtag/Dorian?src=hash&ref_src=twsrc%5Etfw">#Dorian</a>. A prolonged period of hazardous weather conditions that could last for a couple of days is possible across parts of Florida early next week. Visit <a href="https://t.co/tW4KeFW0gB">https://t.co/tW4KeFW0gB</a> for more info. <a href="https://t.co/5n4nGwYNfB">pic.twitter.com/5n4nGwYNfB</a> —@NHC_Atlantic

The storm began Friday over the Atlantic at Category 2, but was already expected to reach Category 3 later in the day, with sustained winds of at least 178 km/h.

The entire state of Florida is under a declaration of emergency and Gov. Ron DeSantis has activated 2,500 National Guard troops, with another 1,500 on standby.

Forecasters predict the storm will grow more ferocious as it slows its advance across the warm waters near the coast, striking land late on Monday or early Tuesday. Tropical storm winds could be felt in Florida as soon as Saturday evening.

No evacuations were ordered as of early Friday, but they're expected as the storm's path become clearer before it makes landfall.

If it reaches Category 4 by Sunday, its winds will blow at more than 210 km/h. Its 9 km/h march across the map could slow down to 6 km/h. The slower it moves, the more time it has to draw fuel from the warm seas.

Recent weather models from the National Hurricane Center show it smacking into the centre of the state. It was trending slightly south in the latest advisory issued at 5 a.m. Friday.

It could roll inland towards Orlando on Tuesday or early Wednesday, weakening as it moves away from the sea. Other NHC weather models show it tracking south toward Miami before it hits the peninsula, or heading north to the Georgia coast.

Along with the dangerous winds, the storm is expected to drop 12 to 25 centimetres of rain on the state, with some areas getting as much as 38 centimetres.

"This rainfall may cause life-threatening flash floods," NHC forecasters said.

U.S. President Donald Trump cancelled a trip to Poland, sending Vice-President Mike Pence in his place, to ensure resources are properly directed for the storm.

"Now it's looking like it could be an absolute monster," Trump said in a video posted on Twitter, adding that food and water were being shipped to Florida.

Hurricane Dorian looks like it will be hitting Florida late Sunday night. Be prepared and please follow State and Federal instructions, it will be a very big Hurricane, perhaps one of the biggest! —@realDonaldTrump

DeSantis said on Friday that local officials are determining evacuation plans.

During the morning briefing, DeSantis said while the precise path of the storm is still uncertain, there is a "high degree of certainty" Dorian will be a major hurricane.

Florida Gov. Ron DeSantis addressing media as Hurricane Dorian approaches the state. (Reuters )

Authorities are urging residents to heed any evacuation orders and be prepared for a "multi-day event," DeSantis said during a news conference.

He also tweeted that people in Florida need to take the storm seriously: "Hurricane #Dorian is moving slowly & gaining strength. Now is the time to get prepared & have a plan."

Georgia Gov. Brian Kemp declared a state of emergency in 12 counties to assist with storm readiness, response and recovery.

'Not looking good'

Category 1-strength winds bend palm trees as Hurricane Dorian slams into St. Thomas, U.S. Virgin Islands, on Wednesday in this still image taken from social media video. (Cassandra Crichlow/Reuters)

Angela Johnson, a 39-year-old bar manager in South Florida, said on Thursday: "We're worried. This is not looking good for us.

"We woke up a lot more scared than we went to bed last night, and the news is not getting any better," said Johnson, who manages Coconuts On The Beach, a bar and restaurant on the surfing beach in the town of Cocoa Beach.

Officials were making piles of sand available for Cocoa Beach residents to fill sandbags starting on Friday.

Dorian could churn across dozens of launchpads owned by NASA, the U.S. Air Force and companies such as Elon Musk's SpaceX and Jeff Bezos's Blue Origin.

CoreLogic, a property information and analytics provider, estimates around 668,000 homes along the east coast of Florida are in the path of Dorian's cone.

It said "$144.6 billion are at potential risk of storm surge damage from Hurricane Dorian based on its projected Category 3 status at landfall."

Hurricane-driven storm surge can cause significant property damage when high winds and low pressure cause water to amass inside the storm, releasing a powerful rush over land when the hurricane moves onshore, the company said.

Gas stations run dry

Some gas stations in Florida have run dry and others have long lines of cars, DeSantis said. He said the state had eased regulations to allow higher-capacity trucks to transport fuel and make it easier to bring in fuel from other states.

Gas prices at the pump are not expected to spike because of the storm, said Patrick DeHaan, head of petroleum analysis at GasBuddy.com.

Local residents form a queue to fill their cars with gas ahead of the arrival of Hurricane Dorian in Kissimmee, Florida, U.S. August 29, 2019. (Gregg Newton/Reuters)

"This is not going to be a pricing event," DeHaan said. "There are no refiners in Florida, there's only a pipeline, and I can't imagine that would be affected significantly. Fuel is flowing, and that's the most important factor."

"We're also going to be, starting today, implementing Florida Highway Patrol escorts for fuel trucks so we can increase fuelling in critical parts of the state."

Colonial Pipeline, which delivers fuels such as gasoline and diesel from the Gulf Coast to markets across the southern and eastern United States, said it was operating normally. It had no plans to shut down ahead of Dorian's projected landfall.

Florida officials also were making sure all nursing homes and assisted living facilities had generators, and were checking with 107 facilities where information about generators was uncertain.

Earlier this week, police in Hollywood, Fla., charged four nursing home caregivers with causing the deaths of 12 patients related to sweltering heat from a post-hurricane power outage left by Hurricane Irma two years ago.