New evacuations were ordered Wednesday as slow-moving Hurricane Isaac caused one levee outside New Orleans to overtop and threatened others. Inside New Orleans, levees and pumps were protecting the city from widespread flooding, but Isaac had cut power to a third of Louisiana's households and was expected to lash the state with heavy rain and winds into Friday.
"We have flooding, inundated four-to-nine feet in areas," parish emergency management official Guy Laigast told the Weather Channel. "We've got homes that have been inundated."
"It's piling that water up on the east side of the Mississippi River," he added. "All that water is ponding up in that area, and that's what's causing the overtopping."
The area had been under a mandatory evacuation order, but only half of the 2,000 residents reportedly had left ahead of Isaac's landfall Tuesday.
By Wednesday afternoon, the threat eased on the east side of Plaquemines Parish, but a shift in the wind now threatens the west side, triggering mandatory evacuations there of some 3,000 residents -- among them 112 nursing home residents.
With the wind having shifted, officials were also looking at whether to deliberately breach the overtopped levee so that water flushes out more quickly.
As a storm surge threatens flood walls in Plaquemines Parish, La., the National Guard is working to rescue dozens of people at risk in the area. NBC's Gabe Gutierrez reports.
Mandatory evacuations were also ordered for parts of St. John Parish, Gov. Bobby Jindal said at a press conference.
"We will be dealing with this storm through early Friday," he said, adding that Isaac's eye wouldn't reach Baton Rouge until Wednesday evening.
The storm surge also flooded areas of the Mississippi coast with water rising several feet in some parts, authorities said. Weather Channel meteorologist Paul Goodloe reported a number of homes had been flooded in Biloxi Bay, Miss.
"The entire stretch of U.S. 90 has been closed from the Bay St. Louis Bridge to the Biloxi Bay Bridge" due to flooding, he reported. The surge is close to overtopping the seawall between the beach and a grassy area off U.S. 90, he added. "And, by the way, high tide is coming up in a couple of hours."
Three adults and an infant stuck on a house boat in Hancock County, Miss., had to be rescued, NBC News reported, as low-lying areas were inundated with water.
In southern Louisiana, more than 656,000 households, or 31 percent of the state, were without electricity Wednesday afternoon due to downed power lines, Jindal said.
Some 4,000 people were in shelters, he added.
The Weather Channel's Jim Cantore and Stephanie Abrams, along with TODAY's Al Roker, reports from New Orleans, where Hurricane Isaac has made landfall.
Worst hit has been Plaquemines Parish, where images on Wednesday showed many homes under water.
"It's something I've never seen before," Parish President Billy Nungesser told the Weather Channel of the flooding there. "I rode out Katrina, and my home has more damage now. It has not let up one time throughout this whole event. The driving rain, all the telephone poles down in Plaquemines Parish. This is not a Category 1 storm."
The National Weather Service has issued flash flood warnings for Orleans Parish, which includes New Orleans, Jefferson Parish, East Bank of Plaquemines Parish, Northwestern Plaquemines Parish, Western St. Bernard Parish and St. Charles Parish in Louisiana and Jackson County, Miss.
Rick Knabb, director of the National Hurricane Center, told The Weather Channel that the storm's large size meant it was "not going to fall apart real quick."
TODAY's Al Roker reports from New Orleans, La., where Hurricane Isaac has roared ashore with 80 mph winds.
"So much of the circulation is over water, and so close to or over the warm waters of the Gulf of Mexico, there's really no reason why (Isaac) can't tap into the energy that warm water provides," he added.
"The large size means it's a bigger storm surge producer than a smaller Category 1," he noted. "It's slow motion means that the water is going to be piling up all day long and it's not going to go out today."
Combined with Isaac's slow movement, this meant there would be "elevated [seawater] surge levels all day today," he added.
In New Orleans, Mayor Mitch Landrieu ordered a dusk-to-dawn curfew overnight as a precaution, and said the post-Katrina investment in beefed-up levees and pumps had paid off.
"It's holding up," he told NBC News. "There's no risk of any failure from what we can tell, anywhere."
Some 60 percent of the city was without power, however, he said.
"Of course, the longer the rain and the longer the wind, the greater the possibility of street flood in interior streets," Landrieu said earlier. "That wind is really, really heavy, which is why it is important that you stay inside."
"There are a lot of trees that are down," he added. "We have reports of streets being flooded in the city."
Knabb also warned that isolated areas would get up to 20 inches of rain with 7 to 14 inches falling over a widespread area. "We're going to see flooding out of this from the freshwater perspective" in addition to the seawater storm surge, he said.
The center of Isaac first made landfall Tuesday evening with 80-mph winds and then moved back over water before making a second landfall just west of Port Fourchon, La., around 2:15 a.m. local time (3:15 a.m. ET).
By late Wednesday morning, it had weakened slightly to 75 mph.
In Mississippi, Hancock County Emergency Management Director Brian Adam said early Wednesday that water stood up to 4 feet deep in the most vulnerable areas.
NBC's Janet Shamlian reports from Pass Christian, Miss., where Hurricane Isaac has completely flooded roads and destroyed a marina.
"Were still experiencing heavy winds, heavy rains" that should last overnight, he told NBC News. "We have road flooding in low-lying areas. ... We have about 7,200 without power."
"We're kind of hunkered down to the point where we're not doing a whole lot of traveling," he added.
Tornado warnings have been posted because of thunderstorms in southern Mississippi. But back streets of some communities showed little evidence of Isaac's passage save for occasional downed tree branches.
No discernible damage to refineries or offshore oil and gas platforms was reported.
NBC's Michael Brunker as well as The Associated Press and Reuters contributed to this report.
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