New Orleans residents fill sand bags on Friday, in preparation for Hurricane Nate. (Sean Gardner/Getty Images)

Nate was upgraded to a hurricane late Friday as the central Gulf Coast braced for its landfall as early as Saturday evening, with lashing winds and storm surges forecast to hit a part of the coast that had largely been spared in this extraordinarily busy hurricane season.

On Saturday, the National Hurricane Center reported Nate had maximum sustained winds at 80 mph, and could gain force as it hurtled toward the Gulf Coast.

“An Air Force Reserve Hurricane Hunter aircraft just penetrated the center of Nate and reported hurricane-force winds,” the center said.

“Reports from Air Force aircraft indicate that maximum sustained winds have increased to near 80 mph with higher gusts. Additional strengthening is expected through Saturday up until the time Nate makes landfall along the northern Gulf Coast.”

The storm, already blamed for at least 22 deaths in Nicaragua and Costa Rica this week, swelled as it crossed unusually warm waters in the Gulf of Mexico. New Orleans officials ordered mandatory evacuations of three low-lying areas of the city.

Sand bags are place atop a levee near the pumping station in New Orleans on Friday. (Sean Gardner/Getty Images)

The center put coastal Louisiana, Mississippi and Alabama up to the Florida border under a hurricane warning, with a storm surge warning in effect from Morgan City, La. — west of New Orleans — to Walton County, Fla. Rain bands are likely to strike the coast as early as Saturday afternoon.

If the storm surge occurs at the time of high tide, water levels could rise as much as 9 feet above ground level in the area stretching from the mouth of the Mississippi River across to the Alabama-Florida border, the center said. It added that the deepest water would be along the coast, east of the landfall location, where the surge would be accompanied by “large and destructive waves.”

It forecast up to 10 inches of rain east of the Mississippi River from the central Gulf Coast into the Deep South, eastern Tennessee Valley, and southern Appalachians.

Here in the Crescent City, where memories of the 2005 Katrina catastrophe remain vivid, officials have acknowledged that their hobbled network of pumping stations could be overmatched by Nate’s downpours.

But Mayor Mitch Landrieu sounded upbeat and resolute in a news conference Friday afternoon, even as he announced a curfew starting at 7 p.m. Saturday and lasting until Sunday morning.

“We are ready for whatever Nate brings our way,” he said. “If we all stay informed, if we all stay alert, if we all stayed prepared, ultimately we will all be safe.”

The city’s drainage system has been challenged even by typical summer storms. Parts of New Orleans have flooded several times this year, including as recently as last week. The worst flooding ­occurred after torrential rains Aug. 5, when up to nine inches fell in just a few hours.

The city’s drainage system is “terribly underfunded,” Landrieu said.

“It’s old. It’s tired. It’s like your grandmother’s car that’s got 400,000 miles on it,” he said. “The pumping system in the city of New Orleans is as old as Calvin Coolidge.”

Landrieu said 109 of the city’s 120 drainage pumps were operational, with contractors working round-the-clock to repair the remainder. Inland flooding from rain is not the primary threat from Nate, he noted, but rather the storm surge along the coast, and most of all the wind, which he warned could turn objects left outdoors into dangerous projectiles.

He reminded residents that they should not drive through underpasses that flood readily. He said they can park their vehicles where they find higher ground without fear of parking tickets — enforcement of violations will be suspended starting at 8 a.m. Saturday.

By Saturday, the Port of New Orleans will be closed, and most of the 200 floodgates in the city and surrounding parishes will be closed. More than 350 members of the National Guard will be on the ground.

Friday morning, the city began providing 17,000 sandbags to residents at five locations across town. Police set up 146 barricades in flood-prone areas, and boats and high-water vehicles were lined up at fire and police stations.

Melonie Stewart, customer service director at Entergy, New Orleans’s sole energy provider, warned residents to be prepared for up to seven days without power from the grid.

Nate appears most likely to hit the Gulf Coast to the east of New Orleans. It could deliver a storm surge of four to seven feet above normally dry land, forecasters said. Wherever the storm makes landfall, areas east of the eye will experience stronger winds than those to the west.

Nate is then expected to weaken and travel northeast into the southern Appalachians, where flash floods Sunday and Monday are a serious risk. The storm’s remnants are then likely to head toward the Mid-Atlantic and the Northeast.

The last hurricane to strike this part of the Gulf Coast directly was Isaac, also a Category 1, in August 2012. It left hundreds of thousands of utility customers without power.

Under sunny skies Friday, New Orleans did not appear to be in a state of alarm. Stores were cleaned out of bottled water, and residents filled sandbags provided by municipal authorities, but this was clearly not a city trembling in advance of Nate.

On Desire Street on Friday afternoon, François Robichaux, 45, lay on his belly on the sidewalk, his left arm elbow-deep in a hole. He was not clearing out a catch basin, but rather fidgeting with a broken water meter on the property of a house he is renovating.

Robichaux wasn’t doing much to prepare for the storm. “We usually evacuate, but this one came so fast,” he said. He was confident that he would be safe in his three-story home. “It’s a Category 1, so I’m not worried,” he said.

The forecast includes a significant chance of the storm’s growing into a significant hurricane — even a Category 3, one with sustained winds of up to 129 mph.

The storm Friday was moving north over very warm water, which drives intensification. But that trajectory also meant interaction with Mexico’s Yucatán Peninsula, with pockets of dry air and shearing winds that could enfeeble the storm.

The fast movement of the storm means it is unlikely to drop massive amounts of rain as Hurricane Harvey did while loitering in the Houston area of the Texas coast in August.

The National Weather Service’s hurricane warning, issued Friday, extends from Grand Isle, La. — which is due south of New Orleans — to the Alabama-Florida border.

Samenow and Achenbach reported from Washington. Patricia Sullivan in Washington and Capital Weather Gang meteorologist Brian McNoldy contributed to this report.