Hurricane Ida, which made landfall as a fierce Category 4 storm, plowed through the Gulf of Mexico into Louisiana on Sunday, lashing the coast with 150 mile-per-hour winds, torrential downpours and pounding surf that plunged much of the shoreline under several feet of water.

With communities in the most vulnerable coastal areas ordered to evacuate in advance, residents riding out the storm in their homes braced for the toughest test yet of major upgrades to a levee system constructed following devastating floods in 2005 from Hurricane Katrina.

Sixteen years to the day after Katrina made landfall, Ida slammed ashore around noon near Port Fourchon, Louisiana, a hub of the Gulf's offshore energy industry, blasting the coast with hurricane-force winds extending 50 miles (80 km) out from the eye of the storm.

Less than 100 miles inland to the north, flash-flood warnings were issued for downtown New Orleans, where emergency medical services were suspended earlier in the day in the most populous city of a state already reeling from a fourth wave of COVID-19 infections and hospitalizations.

"I almost found myself in a panic attack when news announced this was the anniversary of Katrina," Janet Rucker, a lifelong New Orleans resident and recently retired sales manager who evacuated to a downtown hotel with her dog, Duece, on Friday night. "This is just not good for our nerves and our psyche."

Farren Clark, an assistant professor at Nicholls State University who studied Katrina's impact and was riding out the storm at his mother's home in Thibodaux, Louisiana, called the arrival of Ida "nerve-wracking."

"I can hear the howl of the storm getting stronger," he told Reuters by phone. "Having done research on Katrina, it is a little bit triggering."

Officials of the U.S. Army Corps of Engineers said they expected the city's newly reinforced levees to hold, though they said they said the flood walls could be overtopped in some places.

"This is one of the strongest storms to make landfall here in modern times," Louisiana Governor John Bel Edwards said at a news briefing.

Hundreds of miles of new levees were built around New Orleans after flooding from Katrina inundated much of the low-lying city, especially historically Black neighborhoods. That monster storm claimed more than 1,800 lives.

Edwards voiced confidence in the billions of dollars in levee improvements since then, saying they were "built for this moment."

Widespread outages

Power outages were widespread in the first hours of the storm, with more than 530,000 Louisiana homes and businesses losing electricity by late in the day, according to state utility Entergy Louisiana.

"As soon the storm passes, we're going to put the country's full might behind the rescue and recovery," President Joe Biden said after a briefing at the headquarters of the Federal Emergency Management Agency in Washington.

Biden earlier said 500 federal emergency response workers were posted in Texas and Louisiana to respond to the storm.

Just three days after emerging as a tropical storm in the Caribbean Sea, Ida had intensified into a Category 4 hurricane and swept ashore with top sustained winds of 150 miles per hour (240 km per hour), the National Hurricane Center (NHC) reported.

Within hours, as Ida crept closer to New Orleans, its sustained winds had diminished to 125 mph, ranking it as a Category 3 storm but one still considered a life-threatening major hurricane, according to the NHC.

Palm trees trembled as rain blasted in sideways through New Orleans on Sunday, where retired 68-year-old Robert Ruffin had evacuated with his family to a downtown hotel.

"I thought it was safer," he said. "It's double trouble this time because of COVID-19."

Hours later, howling winds sucked out windows on the hotel's third floor, and blue curtains were seen fluttering outside.

Inundation from Ida's storm surge - high surf driven by the hurricane's winds - was reported to be exceeding predicted levels of 6 feet (1.83 m) along parts of the coast. Videos posted on social media showed storm surge flooding had transformed sections of Highway 90 along the Louisiana and Mississippi coast into a choppy river.

The NHC also warned of potentially catastrophic wind damage and up to 2 feet (61 cm) of rainfall in some areas.

The governor warned it could take 72 hours for emergency responders to arrive to hard-hit places. Some parishes imposed curfews beginning Sunday evening.

"We're as prepared as we can be, but we're worried about those levees," said Kirk Lepine, president of Plaquemines Parish, home to 23,000 people and one of the most vulnerable areas along Gulf Coast. Lepine said he feared water topping levees along Highway 23.

"That's our one road in and out," he said.

Officials had ordered widespread evacuations of low-lying and coastal areas, jamming highways and depleting gasoline supplies as residents and vacationers fled, although Edwards said it was impossible to evacuate hospitals.

Louisiana hospitals were treating some 2,450 COVID-19 patients after the latest surge of infections, Edwards said, with many already nearing capacity.

"Everyone who cares about New Orleans is worried," said Andy Horowitz, a history professor who wrote "Katrina: A History, 1915-2015." Horowitz fled to Alabama with his family from their home near New Orleans' French Quarter.

Port Fourchon is home to the Louisiana Offshore Oil Port, the country's largest privately owned crude oil terminal.

The Bureau of Safety and Environmental Enforcement (BSEE) said nearly 300 offshore oil and gas platforms were evacuated, cutting Gulf-based petroleum and natural gas production by 96% and 94%, respectively.