In Alabama, Republican Governor Kay Ivey urged residents in areas facing heavy winds and storm surges to take precautions.
Five ports along the Gulf Coast were closed to shipping as a precaution.
Most oil and gas platforms in the US Gulf of Mexico evacuated their staff and stopped production ahead of the storm.
In an update at 03:00 GMT, the NHC said a hurricane warning was in effect for the “mouth of the Pearl River to the Alabama-Florida border”.
Evacuation orders were put in place for some low-lying areas.
Louisiana Governor John Bel Edwards declared a state of emergency ahead of the hurricane, saying more than 1,000 National Guard troops had been mobilized with a number sent to New Orleans to monitor the drainage pumps there.
A mandatory curfew from 18:00 local time was in place in New Orleans, where residents from areas outside the city’s levee system were evacuated.
A tropical storm warning is currently in effect for New Orleans.
The NHC said that Hurricane Nate “is expected to weaken quickly after landfall, and it is likely to become a tropical storm on October 8.
“It should degenerate into a remnant low late Monday.”
Nate went past Mexico’s Yucatan Peninsula – home to the popular beach resorts of Cancun and Playa del Carmen – on Friday night as it headed north, the NHC said.
Nate caused heavy rains, landslides and floods which blocked roads, destroyed bridges and damaged houses as it tore through central America.
At least 13 people died in Nicaragua, eight in Costa Rica, three in Honduras and one in El Salvador.
The tail of the storm is still causing problems in the region, where thousands have been forced to sleep in shelters and some 400,000 people in Costa Rica were reported to be without running water.
Puerto Rico’s government has asked a judge to defer key deadlines in its bankruptcy case as it grapples with Maria’s devastation.
President Trump came under fire after he spent the weekend focusing on a feud with NFL players and coaches, instead of acknowledging the Puerto Rico disaster.
He tweeted about the crisis on September 25 – but angered critics by suggesting that Puerto Rico’s $72 billion debt needed to be addressed amid its appeal for relief aid: “Texas & Florida are doing great but Puerto Rico, which was already suffering from broken infrastructure & massive debt, is in deep trouble owed to Wall Street and the banks which, sadly, must be dealt with. Food, water and medical are top priorities – and doing well. #FEMA.”
On September 26, the White House announced that President Trump had increased federal funding and assistance for debris removal and emergency protective measures in Puerto Rico in the aftermath of the storm.
The president said he would visit Puerto Rico on October 3 – nearly a fortnight after the storm struck – because it was the “earliest I can go without disrupting relief efforts”.
President Trump may also visit the US Virgin Islands, which was hit by both Hurricane Maria and Irma.
He said his administration was doing a “really good job” and that the Governor of Puerto Rico, Ricardo Rossello, was “so grateful”.
President Trump said rescue efforts were complicated because Puerto Rico was offshore.
“This is a thing called the Atlantic ocean, this is tough stuff,” he said.
He added: “Puerto Rico is very important to me. The people are fantastic. I grew up in New York so I know many Puerto Rican people.”
Ricardo Rossello told Reuters President Trump’s handling of the disaster had been “excellent” and that the government had “responded very quickly” to the crisis.
More than 10,000 US Federal Emergency Management Agency (FEMA) staff are on the ground in Puerto Rico and the US Virgin Islands to assist with relief efforts, according to the agency.
Earlier, Puerto Rico Governor Ricardo Rossello asked President Donald Trump to declare the island a disaster area after the storm unleashed heavy flooding and life-threatening winds.
The governor said major damage was inevitable, although 500 shelters had been established to protect people.
Hurricane Maria has already left seven people dead on the island of Dominica, which was badly affected on September 18.
Aerial footage shows flattened houses and the death toll on Dominica is likely to rise, with details remaining scant as communication links are down.
Images shared on social media show roofs being stripped away as winds as strong as 140 mph whipped trees and power lines in Puerto Rico’s capital city, San Juan.
Puerto Rico is facing $73 billionn in public debt and the damage left by Maria could exacerbate its financial crisis. Declaring a disaster would mean the island could receive more federal assistance in its recovery.
According to the US National Hurricane Center (NHC), Maria made landfall in Yabucoa in the east of Puerto Rico on September 20 as a category 4 storm. It was the first category 4 hurricane to directly hit Puerto Rico since 1932.
Hours earlier, Maria barreled through the US Virgin Islands’ St Croix as a category 5 storm, sustaining winds of up to 175mph.
There was flooding in the French territory of Guadeloupe, where one person was killed by a falling tree and another died on the seafront. At least two others were missing after their ship sank near Desirade, the easternmost island in the archipelago.
Hurricane Maria is the second devastating storm to hit the Caribbean this season – the first being category 5 Irma earlier in September. Maria began moving roughly along the same track as Irma.
Officials in Puerto Rico feared the debris left by Irma could prove extremely dangerous in the high winds.
Maria’s eye is 140 miles north-east of Barbados, and Maria is moving west-northwest at about 13mph.
The NHC says: “On the forecast track, the centre of Maria will move across the Leeward Islands late Monday and Monday night and then over the extreme north-eastern Caribbean Sea Tuesday and Tuesday night.”
The most southerly point of the Leeward Islands – where Maria will first strike – include Antigua and Barbuda. The latter island was evacuated after being devastated by Irma.
The NHC says that “a dangerous storm surge accompanied by large and destructive waves will raise water levels by as much as 5-7ft above normal tide levels near where the centre of Maria moves across the Leeward Islands”.
It also forecasts a maximum potential rainfall of 20in across the central and southern Leeward Islands – including Puerto Rico and the US and British Virgin Islands – through to Wednesday night.
“Rainfall on all of these islands could cause life-threatening flash floods and mudslides,” it warned.
Earlier this month, Hurricane Irma left more than two-thirds of homes on the Dutch side of the island of St Martin (known as Sint Maarten) uninhabitable, with no electricity, gas or drinking water.
The French government has said its side of St Martin – known as Saint-Martin – sustained about €1.2 billion ($1.44 billion) in damage, with nine deaths across Saint-Martin and nearby St Barts.
On the British Virgin Islands, entire neighborhoods were flattened.
After a visit to the area, British Foreign Secretary Boris Johnson described the damage as something “you see in images of from the First World War”.
Virgin boss Richard Branson, who has a home in the Virgin Islands, has been tweeting ahead of the storm’s predicted arrival, warning people to stay safe.
Hurricane Irma also hit the US, with 11 deaths being linked to the hurricane. Nearly 6.9 million homes were left without power in Florida, Georgia, North Carolina, South Carolina and Alabama.
A second hurricane, Jose, is also active in the Atlantic, with maximum sustained winds of 90mph.
The center of the storm was about 335 miles south-east of Cape Hatteras in North Carolina, the NHC said in its advisory on September 17.
Tropical storm watches have been issued for parts of the north-eastern US.
Brock Long, the administrator of Federal Emergency Management Agency (FEMA), has said that Hurricane Irma will “devastate” either Florida or neighboring states.
He said parts of Florida would be without power for days. Half a million people in the state have been ordered to leave their homes.
Hurricane Irma has left a trail of destruction in the Caribbean, affecting an estimated 1.2 million people.
At least 20 people are known to have died so far.
Irma has been downgraded to a category 4 storm, but officials warn that it remains “extremely dangerous”.
According to the US National Weather Service, Irma was expected to bring wind speeds of around 165mph over the weekend as it hits Florida.
Brock Long said: “Hurricane Irma continues to be a threat that is going to devastate the United States in either Florida or some of the south-eastern states.”
“The entire south-eastern United States better wake up and pay attention,” he added.
The death toll continued to rise on September 8 in the Caribbean.
France’s Interior Minister Gérard Collomb said nine people were dead and seven missing in the French territory on St Martin, an island shared with the Netherlands, and St Barthélemy, known more commonly as St Barts. Another death – the second – has been confirmed in the Dutch territory of Sint Maarten.
French officials said six out of 10 homes on Saint-Martin were so badly damaged that they were uninhabitable.
The US Consulate General in Curacao said it believes an estimated 6,000 Americans are stranded on the island.
French, British and Dutch military authorities have deployed aid – including warships and planes equipped with food, water and troops – to their territories.
Irma, which most recently lashed the Turks and Caicos Islands and brought torrential rain to the Dominican Republic and Haiti, was headed towards Cuba and the Bahamas.
The worst of the storm is expected to hit east and central Cuba, with the eye of the storm predicted to pass between the north coast of Cuba and the Bahamas, the National Hurricane Center (NHC) said.
About 50,000 tourists are fleeing or have fled Cuba, with resorts on the north coast now empty, Reuters reports.
A huge evacuation of south-eastern, low-lying coastal areas in the Bahamas has been ordered. The tourism ministry said in a video statement that thousands of tourists left before the storm’s arrival.
Meanwhile Brock Long predicted a “truly devastating” impact on Florida.
South Florida “may be uninhabitable for weeks or months” because of the storm, the US National Weather Service said.
On the archipelago of Turks and Caicos, with its population of about 35,000, one witness described a drop in pressure that could be felt in people’s chests.
Irma ripped off roofs on the capital island, Grand Turk, flooded streets, snapped utility poles and caused a widespread black-out.
Irma also caused some damage to roofs, flooding and power outages in the northern parts of the Dominican Republic and Haiti.
Irma is due to hit Florida as a category 4 hurricane on September 10, bringing storm surges and flooding.
Florida Governor Rick Scott said on September 7: “If you look at the size of this storm, it’s huge.”
“It’s wider than our entire state and could cause major and life-threatening impacts on both coasts – coast to coast.”
President Donald Trump said: “I can say this: Florida is as well prepared as you can be for something like this. Now it’s just a question of what happens.”
Hurricane and storm surge warnings have been issued for south Florida and the Florida Keys, the NHC says.
President Trump’s own Mar-a-Lago estate in Palm Beach is among those ordered to evacuate, the Washington Post tweeted.
After ripping through Florida’s Atlantic coast, Irma is expected to move into Georgia and South Carolina.
Georgia Governor Nathan Deal said a mandatory evacuation on the state’s Atlantic coast was scheduled to begin on September 9.
Roads and airports have been jammed as thousands of people tried to evacuate areas at risk, with reports of fuel shortages and gridlock on some roads.
Many tourists are stranded with no seats left on flights back to their countries as flights to and from airports in Florida are being suspended.
Orlando’s international airport said commercial flights would stop from 17:00 local time on September 9.
Another storm, Jose, further out in the Atlantic behind Irma, is now a category 4 hurricane, with winds of up to 120mph.
Hurricane Katia, in the Gulf of Mexico, has strengthened to a category 2 storm, with winds of up to 85mph. A warning is in effect for the coast of the Mexican state of Veracruz and the storm is expected to make landfall on September 9.
At least nine people died after Hurricane Irma hit Caribbean causing widespread destruction and reducing buildings to rubble.
The small island of Barbuda is said to be “barely habitable”. Officials warn that St Martin is almost destroyed, and the death toll is likely to rise.
Irma, a category five hurricane, the highest possible level, is passing north of Puerto Rico.
Two other storms have strengthened to become hurricanes.
More than half of St Martin’s three million residents were without power as Irma caused heavy downpours and strong winds. Officials have said that power could be cut off for several days.
The most powerful Atlantic storm in a decade had wind speeds of 185mph and was expected to pass near or just north of the coast of the Dominican Republic on September 7.
Hurricane Irma first hit the dual-island nation of Antigua and Barbuda. At least one death, of a child, was reported on Barbuda, where PM Gaston Browne said about 95% of the buildings had suffered some damage.
“It’s absolute devastation,” he said after flying over the island, home to some 1,600 people.
“The island is literally under water. In fact, I’m of the view that, as it stands now, Barbuda is barely habitable.”