Media playback is unsupported on your device Media caption Hurricane Dorian is heading towards Florida - the governor warns of a 'multi-day event'.

A powerful storm threatening the Bahamas and south-eastern coast of the US has grown to category four, the US National Hurricane Center (NHC) says.

Hurricane Dorian has maximum sustained winds of nearly 145mph (225km/h).

It is expected to grow even stronger, its centre potentially crossing the Bahamas before skirting Florida's east coast early next week.

Reports from the Bahamas described tourists scrambling to leave before the closure of the international airport.

States of emergency have been declared in Florida, Georgia, North and South Carolina. Residents have been urged to stock enough food, water and medicine to last at least a week.

Forecasters warn Dorian could be the region's worst storm since category five Hurricane Andrew killed 65 people and destroyed 63,000 homes in 1992.

US President Donald Trump said he was monitoring Dorian, which he described as "an extremely dangerous storm" on Twitter.

Hurricanes, whose strength can range from category 1 to 5 on the Saffir-Simpson scale, tend to get stronger as they move over warm waters like those off Florida.

By the middle of next week, forecasters expect Dorian to shift eastwards, putting the coasts of Georgia and South Carolina at risk.

What's the forecast?

The NHC said "life-threatening storm surge, devastating hurricane-force winds, and heavy rains capable of life-threatening flash floods" are expected to hit the Abaco Islands and Grand Bahama on Sunday.

Jeffrey Simmons, the deputy director of Bahamas' Department of Meteorology, said storm surges of up to 15ft (4.5m) were expected.

The country's National Emergency Management Agency believes that damage could be exacerbated by the fact that the storm's westward motion is forecast to slow - this would keep it over the islands for longer.

A tropical storm watch is also in effect for a 120-mile (193km) stretch of Florida's eastern coast, with hurricane-force winds possible along the state's coast by early next week.

Media playback is unsupported on your device Media caption Hurricane Dorian approaches the Bahamas

The coasts of Georgia, North Carolina and South Carolina are also at risk of strong winds and a "dangerous storm surge" during the middle of next week.

Dorian's exact path toward Florida remains uncertain but millions of people could be affected, as well as holiday attractions such as Walt Disney World and President Donald Trump's Mar-a-Lago resort.

Dorian is expected to drop up to 12in (30cm) of rain on the coastal US, with some areas getting as much as 18in. Tides in the region are already at some of their highest levels of the year, owing to a naturally occurring event.

A new moon, combined with the coming autumn equinox, has created what are known in Florida as "king tides". These are likely to exacerbate dangerous levels of flooding, forecasters warn.

How are the Bahamas preparing?

Bahamas Prime Minister Hubert Minnis has announced an evacuation order for parts of Grand Bahama and the Abaco Islands, both in the north of the archipelago.

All tourists have also been asked to leave over the last few days.

At a press conference, he begged residents to head for the country's main island to escape the "devastating, dangerous storm".

"I want you to remember: homes, houses, structures can be replaced. Lives cannot be replaced," he said, adding that 73,000 people and 21,000 homes were at risk.

Hurricanes A guide to the world's deadliest storms Hurricanes are violent storms that can bring devastation to coastal areas, threatening lives, homes and businesses. Hurricanes develop from thunderstorms, fuelled by warm, moist air as they cross sub-tropical waters.

Warm air rises into the storm. Air swirls in to fill the low pressure in the storm, sucking air in and upwards, reinforcing the low pressure. The storm rotates due to the spin of the earth and energy from the warm ocean increases wind speeds as it builds. When winds reach 119km/h (74mph), it is known as a hurricane - in the Atlantic and Eastern Pacific - or a typhoon in the Western Pacific. "Everybody has a plan until they get punched in the face. Well, we're about to get punched in the face."

Florida Mayor Bob Buckhorn, ahead of Hurricane Irma (2017) The central eye of calmer weather is surrounded by a wall of rainstorms.

This eyewall has the fastest winds below it and violent currents of air rising through it. A mound of water piles up below the eye which is unleashed as the storm reaches land.

These storm surges can cause more damage from flooding than the winds. "Urgent warning about the rapid rise of water on the SW FL coast with the passage of #Irma's eye. MOVE AWAY FROM THE WATER!"

Tweet from the National Hurricane Center The size of hurricanes is mainly measured by the Saffir-Simpson scale - other scales are used in Asia Pacific and Australia. Winds 119-153km/h

Some minor flooding, little structural damage.

Storm surge +1.2m-1.5m Winds 154-177km/h

Roofs and trees could be damaged.

Storm surge +1.8m-2.4m Winds 178-208km/h

Houses suffer damage, severe flooding

Storm surge +2.7m-3.7m Hurricane Sandy (2012) caused $71bn damage in the Caribbean and New York Winds 209-251km/h

Some roofs destroyed and major structural damage to houses.

Storm surge +4m-5.5m Hurricane Ike (2008) hit Caribbean islands and Louisiana and was blamed for at least 195 deaths Winds 252km/h+

Serious damage to buildings, severe flooding further inland.

Storm surge +5.5m Hurricane Irma (2017) caused devastation in Caribbean islands, leaving thousands homeless "For everyone thinking they can ride this storm out, I have news for you: that will be one of the biggest mistakes you can make in your life."

Mayor of New Orleans Ray Nagin ahead of Hurricane Gustav, 2008 Click arrow to proceed Loading ... Swipe to progress Go back to start

How is Florida preparing?

Governor Ron DeSantis has declared a state of emergency for the whole state, as has President Trump. The governor has activated 2,500 National Guard troops, with another 1,500 on standby.

Shoppers in Florida have been queuing around the block to snap up supplies such as medication and fuel. Some petrol stations reported fuel shortages, while a few shops had run out of bottled water.

The coastal city of Miami ordered the removal of electric rental scooters from the streets to avoid any potential hazards.

Officials fear the rental scooters, operated by firms such as Lime, Lyft and Uber's Jump, could be swept away by strong winds, turning them into projectiles.

Image copyright Getty Images Image caption Workers place hurricane shutters over windows in Florida

No immediate mass evacuations have been ordered by state authorities but President Trump, who had warned that Dorian "could be an absolute monster", said a decision could be made on Sunday.

People have been asked to bring their pets with them in case of evacuation. On social media, the names of hotels that accept pets are being shared.

Orlando International Airport announced it was halting commercial flights from 02:00 (06:00 GMT) on Monday "out of an abundance of caution". Tourist resorts in the city remained open, however.

President Trump cancelled a planned trip to Poland because of the storm, sending Vice-President Mike Pence instead.

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