By Josh Levs and Chelsea Carter, CNN
(CNN) - Isaac lost its hurricane status Wednesday afternoon, but its relentless waves of rain brought dangerous storm surge and flooding to much of coastal Louisiana.
The slow-moving system, downgraded to a tropical storm, threatened to keep churning for another day.
Dozens of families that had ignored mandatory evacuation orders in a low-lying area retreated to their attics and roofs and sought rescue amid the howling wind and pounding rain.
The punishing conditions will persist "all day today, into tonight, into tomorrow," said Richard Knabb, director of the National Hurricane Center.
As of 2 p.m. CT, Isaac's maximum sustained winds were at 70 miles per hour -- below the hurricane threshold of 74 mph. [INTERACTIVE: Track Isaac with the WTVR.com Interactive Hurricane Tracker]
The storm's center was about 55 miles south-southeast of Baton Rouge and about 50 miles west-southwest of New Orleans, the hurricane center said.
Isaac was creeping across the region at only 6 miles per hour, giving it a long time to inflict damage.
Gov. Bobby Jindal said there was a report of a fatality in a fire early Wednesday, but officials had not confirmed the report.
Officials were quick to emphasize that the huge federal investments in recent years to avoid a repeat of Hurricane Katrina's horror had worked.
"The system that the country invested in is absolutely paying off," said Louisiana Sen. Mary Landrieu.
Despite the "terrible, determined, and nasty storm," there has been "no massive and catastrophic flooding," Landrieu said, crediting "the extraordinary investments."
"There is no evidence of any (water) overtopping (canals)" in the city, said the senator's brother, New Orleans Mayor Mitch Landrieu. "We have full confidence the levees will hold."
But in nearby Plaquemines Parish, a levee overtopped, creating a kind of flooding the parish did not see even during Hurricane Katrina, which hit seven years ago Wednesday.
More than 150 calls came in to 911 from people wanting to be rescued, said Terry Rutherford, commander of authorities in Plaquemines Parish.
By mid-morning, 75 people had been rescued from flooded homes and rooftops in Braithwaite in Plaquemines Parish, CNN affiliate WWL reported. At least 25 others were awaiting rescue on the parish's east bank, the report said, citing parish President Billy Nungesser.
Emergency management officials reported the overtopping of an 18-mile stretch of the back levee from Braithwaite to White Ditch, according to the the National Weather Service office in New Orleans.
The levee is maintained by the parish and is not part of the federal hurricane protection levee system, according to office.
Katrina breached the levee in two places, the Army Corps of Engineers said.
"It's very unfortunate that people did not heed the warnings," Mary Landrieu told CNN, adding, "our hearts go out" to them.
"Now the Coast Guard's got to go out with winds still gusting 60 to 70 miles per hour in some areas" to save them, she said.
The New Orleans levee system and pump stations were working furiously to deal with the deluge.
The system was rebuilt and reinforced at a cost of $14 billion after it failed when Katrina struck in 2005. Nearly 1,800 people died as a result of that storm, the majority when levees failed and flooded.
Landrieu said she was among some officials who had pushed for Plaquemines Parish to be included in the federally funded projects, but the corps follows a formula "based on economic impact" that leaves out some rural or sparsely populated areas.
However, Jindal said federal dollars were appropriated for that levee since Katrina, but so far not been used for construction, although there had been some kind of construction done on the levee.
Officials were considering intentionally breaching the levee downstream to allow some of the floodwater to flow back out of the inundated area, Jindal said.
Isaac could bring 14 inches of rain across the region, and as much as 20 inches in some areas, including parts of Louisiana, Mississippi and Alabama, the hurricane center had predicted.
The center of the storm "will move farther inland over Louisiana today and tomorrow, and move over southern Arkansas by early Friday," the hurricane center said.
More than 750,000 customers were without power in Arkansas, Louisiana, Mississippi, Alabama and Texas, according to regional power companies.
Some of the most dire conditions were in Plaquemines Parish.
Three adults and an infant in Mississippi were rescued overnight from a houseboat, the state's emergency operations center said.
Isaac "delivered more of a punch than originally thought," said Nungesser, parish president.
"We have reports of people on their roofs, in attics, in 12 to 14 feet of water," he said Wednesday morning.
Some residents saved three people, including a woman who was on her roof, Nungesser said.
Those homes were on the east bank of a levee, but "this storm is going to kick around and deliver the same type of flow to the west bank," he said.
Parish resident Gene Oddo told WWL that he was in his attic with his wife and 18-month-old baby girl.
"The water came up so quick, it looks like we lost everything," he said. He and his wife have drinking water, baby food and other necessities, he said.
"I would rather be here to save what I can, because insurance doesn't cover all that much," Oddo said.
Oddo said the water was above his front door, and he did not expect it to reach the attic. But if it does, "I'm gonna have to shoot a hole in the attic to get up here on the roof."
His neighbors, including a 92-year-old man who refused to leave his home, were in a similar predicament, he said.
"People who went through Katrina are pretty nervous about storms, and large numbers of people have left," Lynn Magnuson, 58, said Tuesday in a CNN iReport.
Magnuson said the Lower 9th Ward, which was hard hit by Katrina, "is pretty empty right now."
About 1,000 National Guard troops and more than 2,900 law enforcement officers are in the city ready to address issues related to the storm, Mayor Landrieu said.
Isaac made its second landfall at about 2 a.m. CT near Port Fourchon, in southeast Louisiana 60 miles southeast of New Orleans, after slamming first into Plaquemines Parish along the coast and then wobbling back over the water near the mouth of the Mississippi River, the National Hurricane Center said in an early morning update.
The storm caused significant surges and flooding, including in some places not directly in Isaac's path. Storm surges of 9.9 feet were reported in Shell Beach, Louisiana, and 6.2 feet in Waveland, Mississippi, according to the hurricane center.
Forecasters predicted water levels would rise between 6 and 12 feet on the coast in Mississippi and southeastern Louisiana alone.
In Biloxi, Mississippi, 50-year-old Alfonso Walker was keeping a close eye on the progress of the 195-mile-wide hurricane.
He watched as a storm surge sent waves crashing over the pier at the IP Biloxi Hotel & Casino.
"I went through Hurricane Camille in 1969 and Hurricane Katrina in 2005, where I lost everything, and every other hurricane in between those two that came through," he said in a CNN iReport.
"So I'm a little concerned."
Isaac, which was a tropical storm last week in the Atlantic Ocean, killed nearly two dozen people in Haiti and the Dominican Republic before starting its journey across the Gulf of Mexico.
On Tuesday, Isaac grew to a Category 1 hurricane. It is significantly weaker than Category 3 Katrina, though forecasters warn it is capable of causing significant flooding.
Isaac earlier prompted some airports to close in New Orleans; Gulfport-Biloxi, Mississippi; and Mobile, Alabama.
Mobile Regional Airport said Wednesday it was reopening runways and resuming some flights, and Pensacola International Airport announced it reopened in the morning, with the first flight landing at 10:01 a.m.
Major ports along the Gulf Coast and the Mississippi River from Baton Rouge to its mouth were closed, according to the U.S. Coast Guard. Amtrak suspended train service to and from New Orleans on Wednesday, and area businesses have come to a standstill.
CNN's Brian Todd, Soledad O'Brien, Ed Lavandera, John Zarrella, Joe Sterling, Anika Chin, Greg Botelho, Mike Ahlers, Aaron Cooper and Ed Payne contributed to this report.