Hurricane Nate has made landfall in the US state of Louisiana as a category one storm with wind gusts of nearly 140 kilometres per hour.
Nate, the fourth major storm to strike the United States in less than two months, killed at least 30 people in Central America before entering the warm waters of the Gulf of Mexico and bearing down on the US South.
The hurricane warning for New Orleans had been changed to a tropical storm warning.
A hurricane warning remained in effect for the Gulf Coast from Grand Isle Louisiana to the Alabama-Florida border, according to the National Weather Service.
"We're in the fight now. The storm is on us," New Orleans Mayor Mitch Landrieu told reporters at a briefing earlier Saturday afternoon, adding that conditions were expected to rapidly deteriorate.
The National Hurricane Centre said on Saturday night (local time) Nate was expected to make a second landfall along the coast of Mississippi on Saturday night and then pass over parts of Mississippi, Alabama and Tennessee.
The storm had weakened slightly and was moving north at 32kph.
Evacuations have been ordered along the central Gulf Coast and people are hunkering down as they wait on the storm.
Facilities closed, National Guard troops mobilised
Earlier, authorities made last-minute preparations as the hurricane intensified and storm surges of up to 3.7 metres were expected.
In Louisiana the National Guard mobilised 1,300 troops and positioned high-water vehicles, boats and even school buses from Baton Rouge to New Orleans to help with rescues.
Louisiana Governor John Bel Edwards said he spoke with President Donald Trump, who assured him the Federal Government was prepared to respond as well.
More than 40 per cent of manned oil- and gas-producing platforms in the Gulf of Mexico were evacuated, according to the Interior Department.
Some airports in southern US states were closed, as were major shipping ports across the central US Gulf Coast.
The US Coast Guard ordered a halt to all traffic beginning at 8:00am local time for several ports in New Orleans, Mississippi, Alabama and Florida.
New Orleans, which sits near the mouth of the Mississippi River, is an important transit point for energy, metals and agricultural commodities moving to overseas and domestic markets.
Gary LaGrange, executive director of trade group Ports Association of Louisiana, said he expected traffic restrictions to be lifted quickly once the fast-moving storm passed.
"It'll be short-lived, based on the projected path and movement of the storm, unless an unlikely event happens such as two vessels colliding," he said.
Vessels were still moving to secure berths at the ports on Saturday morning, he said.
Hurricane Nate killed at least 21 people as it passed across Central America, hitting Nicaragua and Costa Rica.
The storm has already caused oil companies to evacuate workers at 66 production platforms and five rigs in the Gulf of Mexico, according to the US Government.
As of Friday, operators had shut output equal to 1.24 million barrels per day of oil and 1.7 billion cubic feet per day of natural gas production due the storm, it said.
Phillips 66 also halted operations at its Alliance, Louisiana, oil refinery on Saturday.
The refinery is south of New Orleans along the banks of the Mississippi River.