Ida gathered more strength overnight and made landfall near Port Fourchon, Louisiana, at 11.55am (local time), the National Hurricane Center (NHC) said.
It is the toughest test yet for the hundreds of miles of new levees built around New Orleans after the devastation of Hurricane Katrina, which made landfall 16 years ago to the day, inundating historically Black neighborhoods and killing more than 1800 people.
Louisiana Governor John Bel Edwards said Ida could be the state's worst direct hit by a hurricane since the 1850s. More than 122,000 Louisiana homes and businesses had already lost electricity, mostly in the state's southeast, according to the tracking site PowerOutage.
The state is also dealing with the nation's third-highest rate of new coronavirus infections. Hospitals were treating some 2450 Covid-19 patients, Edwards said, with those in many of the state's parishes already nearing capacity.
Just three days after emerging as a tropical storm in the Caribbean Sea, Ida had swelled into a Category 4 hurricane on the five-step Saffir-Simpson scale with top sustained winds of 240 km/h, the NHC said.
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 from their home in the city's east.
"I thought it was safer," he said. "It's double trouble this time because of Covid."
In the state capital of Baton Rouge, Marvin Broome said he had no choice but to stay home because his wife is the mayor, Sharon Weston Broome. The 73-year-old English teacher said in a telephone interview he was busy stashing the families valuables and important papers in a safe part of their home while Mayor Broome dealt with preparations for the city of 224,000.
Predicted storm surges were already happening, exceeding 1.83m in some parts of the coast. Parts of Highway 90 that runs along the Louisiana and Mississippi Gulf Coast had become a choppy river, according to videos posted on social media.
The NHC also warned of potentially catastrophic wind damage and up to 61 cm of rainfall in some areas.
The National Weather Service station in New Orleans urged the many residents who have no interior rooms in their home to move to a closet or bathroom for protection. Some parishes imposed curfews beginning Sunday evening, forbidding people from going outside.
"We're as prepared as we can be, but we're worried about those levees," said Kirk Lepine, president of Plaquemines Parish on the state's Gulf Coast.
Plaquemines, one of the most vulnerable parishes, is home to 23,000 people along the Mississippi delta stretching into the Gulf. Lepine feared water topping the levees along Highway 23.