Hurricane Nate has brought strong winds, torrential rain and a threat of storm surges to the US Gulf Coast.
The storm, with maximum sustained winds of 85mph (137km/h), made landfall near the mouth of the Mississippi River in Louisiana, late on Saturday.
Louisiana, Mississippi, Alabama and parts of Florida earlier issued warnings and evacuation orders amid fears of rapidly rising sea waters.
The tropical storm then strengthened to a category one hurricane on the Saffir-Simpson scale as it headed towards the US.
Although not as strong as last month's Maria and Irma, officials had warned Nate was a fast-moving storm that could bring flooding to low-lying areas.
US President Donald Trump on Saturday issued an emergency declaration for Louisiana, allowing the state to seek federal help with preparation and possible relief efforts.
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".
Louisiana Governor John Bel Edwards declared a state of emergency ahead of the hurricane, saying more than 1,000 National Guard troops had been mobilised with a number sent to New Orleans to monitor the drainage pumps there.
A mandatory curfew from 18:00 (23:00 GMT) was in place in New Orleans, where residents from areas outside the city's levee system were evacuated.
The NHC said that Nate "is expected to weaken quickly after landfall, and it is likely to become a tropical storm Sunday morning.
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.
UN Secretary General Antonio Guterres, visiting Antigua and Barbuda which were badly damaged by Hurricane Irma in September, said the international community needed to do more to help Caribbean countries hit by a series of powerful hurricanes.
"There is an increasing intensity of hurricanes, an increasing frequency, and increasing devastation," he told the BBC. "The origin is clear - we are facing the consequences of climate change."
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