The U.S. Gulf Coast braced Friday for a fast-moving blast of wind, heavy rain and rising water as deadly tropical storm Nate threatened to reach hurricane strength before a weekend landfall.

The Miami-based National Hurricane Center (NHC) issued hurricane and storm surge warnings for southeast Louisiana and the Mississippi and Alabama coasts. States of emergency were declared in all three states as Nate — which has already killed at least 21 people in Central America — became the latest in a succession of destructive storms this hurricane season.

Nate is forecast to dump seven to 15 centimetres of rain on the region — with isolated totals of up to 30 centimetres. That much rain led authorities to warn of flash flooding and mudslides. By midafternoon Friday, Nate was moving at a speed of 33 km/h. Its centre was located about 200 kilometres east-southeast of Cozumel, Mexico, and was expected to reach the U.S. late Saturday or early Sunday.

Evacuation orders were issued for some coastal communities, including the Louisiana towns of Jean Lafitte and Grand Isle.

Shelly Jambon, owner of Sureway Supermarket in Grand Isle, said she plans on riding out the storm at her store even though it's across the street from the beach. She bought it two years before Hurricane Katrina struck in 2005 and has weathered far more threatening storms than Nate.

"It's a mild one for us," she said. "Seventy to 80 mile per hour winds? We get that in a winter storm."

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Tropical storm Nate caused mudslides in Costa Rica, damaging homes like this one in San Jose. (Juan Carlos Ulate/Reuters)

State of emergency

Louisiana  mobilized 1,300 National Guard troops. Some were headed to New Orleans, where summer storms already have exposed problems with the city's fragile pumping system.

"We don't anticipate that this is going to cause a devastating impact to New Orleans or exceed the ability for the pumps," Gov. Jon Bel Edwards said Thursday.

Mississippi Gov. Phil Bryant declared a state of emergency in six southernmost counties. State officials, at a briefing Friday in Gulfport, warned that Nate's main danger in that state will be from up to three metres of storm surge in low-lying coastal areas, as well as from winds that could damage mobile homes.

"If you are in an area that has flooded, I would recommend you evacuate that area until the storm has ended and the water has receded for your own personal safety and for the safety of the first responders that will be responding in the event you are trapped," Bryant said.

The storm threatened to disrupt one of the Mississippi coast's biggest annual tourist events, the "Cruisin' the Coast" auto show. Biloxi firefighters warned more than 700 recreational vehicle campers that they may need to leave early. The event continued as normal Friday, but Saturday's events were cancelled, replaced by a brief closing ceremony.

'Fast-moving storm'

Dozens of offshore oil and gas platforms and drilling rigs in the Gulf of Mexico have been evacuated as Nate churns through warm waters. Ingalls Shipbuilding, the Mississippi coast's largest industrial employer, announced Friday that only a skeleton crew of necessary employees would work Saturday and Sunday at the Pascagoula shipyard.

The northern Gulf Coast areas targeted by Nate largely have been spared the worst effects of a catastrophic hurricane season, but Louisiana's emergency declaration for Nate isn't its first since the start of the summer. In August, a weakened Hurricane Harvey made landfall in Louisiana after dealing a devastating blow to Texas and then nudging back into the Gulf of Mexico. Edwards also issued an emergency declaration in August for storm-related flooding in New Orleans.

On Alabama's Dauphin Island, owners hauled boats out of the water ahead of the storm's approach. Tourists canceled beach reservations for the weekend. The major concern was that Nate's storm surge was projected to coincide with high tide.

Mississippi Emergency Management Agency Director Lee Smithson expressed confidence that the federal government would be able to provide help to Mississippi even as the Federal Emergency Management Agency continues to respond to previous hurricanes in Texas, Florida, Puerto Rico and the Virgin Islands.

Bryant authorized the use of the Mississippi National Guard to respond to any damage. Officials said they would open 11 evacuation shelters in areas away from the immediate coast, and that the regional bus system could transport people who can't drive to shelters on their own.

"This is a fast-moving storm," Smithson said.

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A man surveys rising waters on the Masachapa river on the outskirts of Managua, Nicaragua, after tropical storm Nate. (Oswaldo Rivas/Reuters)