(Adds quotes, details from Louisiana)
NEW ORLEANS, Oct 7 (Reuters) - Hurricane Nate strengthened on Saturday as it churned through the Gulf of Mexico, threatening to power into the U.S. central Gulf Coast to the east of New Orleans as a Category 2 storm after killing at least 30 people in Central America.
The center of the hurricane, the fourth major storm to hit the United States in less than two months, is forecast to make landfall overnight between Slidell, Louisiana, and Alabama's Mobile Bay, U.S. National Hurricane Center spokesman Dennis Feltgen said.
Nate's outer bands will likely reach the U.S. Gulf Coast with tropical storm-force winds on Saturday evening, the Miami-based NHC said.
Currently a Category 1 hurricane, Nate was 105 miles (170 km) south of the mouth of the Mississippi River on Saturday afternoon and moving at a rapid 25 miles per hour (40 km per hour), the NHC said. Maximum sustained winds hovered just below 90 mph (145 kph), with gusts of up to 110 mph.
The NHC issued a hurricane warning from Grand Isle, Louisiana to the Alabama-Florida border. A state of emergency was declared for more than two dozen Florida counties and for the states of Alabama, Louisiana and Mississippi.
New Orleans, 30 miles (48 km) southwest of Slidell, evacuated some residents from areas outside its levee system as the storm approached. The winds could cause significant power outages in the city, and bring storm surges as high as 6 to 9 feet (1.8 to 2.7 meters), Mayor Mitch Landrieu said.
"We have been through this many, many times. There is no need to panic," Landrieu told a news conference, alluding in part to Hurricane Katrina, which triggered severe flooding in New Orleans and killed hundreds of people in August 2005.
But residents of the city known as the "Big Easy" were taking Nate in stride. At a Lowe's hardware store in the St. Roch area of New Orleans, there were short lineups around midday and plentiful supplies of propane, generators and plywood.
"They don't start boarding up until it's a Category 3," said employee Paula Clemons. "We're used to floods. Comes with the territory."
That said, for some residents of New Orleans, memories of Katrina and Hurricane Betsy in 1965 were still vivid.
By Saturday afternoon, as Nate's outer band pelted sheets of rain on the city, residents had filled 13,000 sandbags at a fire hall on Elysian Fields Drive, just one of five such sandbag depots in the city.
"I've been through Betsy and Katrina. Ain't no way they're going to stop this water," said Antoine Turner, 55, as he heaved sandbags into his half-ton truck, hoping to protect a building where he was preparing to open a soul food restaurant. "Best thing to do is just pray."
In Belle Chasse, a town 10 miles south of New Orleans that was flooded by a storm surge during Katrina, neighbors Derrick Ulloa, 27, and Ryan Hunt, 28, faced a 20-foot concrete wall designed to keep the Mississippi River off their doorsteps.
"We're not worried about it," said Hunt, as he and Ulloa organized jerry cans of gasoline to power a generator that will keep their fridges and an air conditioner running through the storm.
After hitting the U.S. Gulf Coast, the storm is likely to veer to the northeast and cut through Alabama, the state likely to be hit hardest. Republican Governor Kay Ivey urged residents in areas facing heavy winds and storm surges to take precautions.
Between four and eight inches of rain will fall from far southern Mississippi and northern and western Alabama to northern Georgia, middle and eastern Tennessee, western North Carolina and the Virginia Panhandle, AccuWeather forecast.
Nate will mark the fourth major storm to slam the United States in the current hurricane season, following Harvey, Irma and Maria, which devastated Texas, Florida and Puerto Rico, respectively.
But as a Category 1 or 2, the weakest in the five-category ranking used by meteorologists, Nate may not pack the same punch as its predecessors.
Major shipping ports across the central U.S. Gulf Coast were closed to inbound and outbound traffic on Saturday, as Nate intensified and storm surges of up 11 feet (3.74 meters) were expected at the mouth of the Mississippi River.
The U.S. Coast Guard ordered the closure of the ports of New Orleans; Gulfport and Pascagoula, Mississippi; Mobile, Alabama, and Pensacola and Panama City, Florida.
The storm has curtailed 92 percent of daily oil production and 77 percent of daily natural gas output in the Gulf of Mexico, more than three times the amount affected by Harvey. Workers had been evacuated from 301 platforms and 13 rigs as of Saturday, the U.S. Bureau of Safety and Environmental Enforcement.
Before heading north into the Gulf, Nate brushed Mexico's Yucatan peninsula, home to beach resorts such as Cancun and Playa del Carmen, the NHC said.
The storm doused Central America with heavy rains on Thursday, killing at least 16 people in Nicaragua, 10 in Costa Rica, two in Honduras and two in El Salvador.
Thousands were forced to evacuate their homes and Costa Rica's government declared a state of emergency.
(Additional reporting by Brendan O'Brien in Milwaukee, Alex Dobuzinskis in Los Angeles, Oswaldo Rivas in Managua, Erwin Seba and Gary McWilliams in Houston; Editing by Matthew Lewis, Diane Craft and Paul Simao)