Isaac continues to spin slowly through Louisiana, pushing huge amounts of sea water, dumping torrents of rain and leaving locals scrambling up to attics and onto roofs as floodwaters threaten to overtake them.
"It's our own little Katrina," said Tania Trege, wife of St. John the Baptist Parish Sheriff Mike Trege, describing the situation in Laplace, La.
Officials there already have rescued 200 people from flooded homes and believe hundreds more are stranded. The sheriff's office said rescuers continue to try to reach stranded locals with flat-bottomed boats and other vessels in what was described as the worst flooding in decades -- even worse than that caused locally by Katrina seven years ago.
State officials said there are as many as 3,000 people being evacuated from Laplace, with 40 to 80 buses waiting to take people out.
Overall, more than 700,000 people have been left without power in four states as Isaac, now a tropical storm, continues to pummel the Gulf Coast with rain and maximum sustained winds of 60 mph.
Forecasters at NOAA warned that Isaac could create "life-threatening hazards from storm surge and inland flooding as it moves slowly across southeastern Louisiana."
As of 10 p.m. CT, Isaac's center was located about 70 miles west-northwest of New Orleans and 15 miles south of Baton Rouge, La., and it was moving northwest at approximately 6 mph. Its center was expected to remain in Louisiana until early Friday, when it was forecast to cross into Arkansas shortly after gradually weakening into a tropical depression.
New Orleans Mayor Mitch Landrieu declared a curfew in the city in order to make things easier for utility crews working through the night, The Associated Press reported.
An unofficial rainfall total of 22.5 inches was reported in Arabi, La., near the city's 9th Ward. An official report from Audubon Park in New Orleans listed 17 inches of rainfall.
Three helicopters were in the New Orleans area in case residents needed to be rescued from floodwaters. Each crew was equipped with hoist capability and a rescue swimmer, according to Coast Guard officials.
Hurricane Isaac weakened into a tropical storm this afternoon -- but not before its powerful storm surge overtopped levees, raising water levels as far as 314 miles up the Mississippi River.
As of 10 p.m. CT, a tropical storm warning remained in effect from Cameron, La., to the Mississippi-Alabama border, though the range and magnitude of the warning had slowly decreased.
Despite the downgrade, forecasters said Isaac wasn't running out of steam just yet.
Five to 10-foot storm surges were expected in Mississippi and southeastern Louisiana, with 7 to 14 inches of rainfall.
The Central Gulf Coast region and part of the Lower Mississippi Valley could experience tornados through Thursday.
Isaac, which at its peak was a weak Category 1 hurricane, showed storm surge heights more characteristic of a strong Category 2 storm.
The hurricane overtopped levees, knocked down trees and cut power to 716,068 in Alabama, Florida, Louisiana and Mississippi
There were no reports of injuries but dozens of residents of Plaquemines Parish, La., were stranded atop a levee, while there were multiple reports of people trapped in attics by rising waters.
As of mid afternoon, fewer people had been evacuated than during Hurricane Katrina, which hit New Orleans seven years ago today.
Nevertheless, speaking with ABC News affiliate WFAA at the scene of a water rescue conducted by the local fire department in Plaquemines Parish, Cheryl Hicks said that the waves crested above her head.
"It's over 20 feet," she said. "It is horrible. Everybody's home is gone. Nobody has a house in Braithwaite. Nobody.
"This is a g**damn shame," Hicks added. "I've lived in Braithwaite for 53 years and this is my first time seeing something like this. This hasn't ever happened to us. This is a shame."
A total of 56 parishes in Louisiana declared states of emergency, according to Louisiana Gov. Bobby Jindal, who added that there was a breakaway of a non-federal levee on the East Bank in Plaquemines Parish. The parish had a mandatory evacuation at noon Tuesday.
Officials were considering conducting an intentional breach to release some of the water at that levee.
At 9 a.m., 30 to 40 vehicles were stranded atop the levee in Plaquemines waiting for a ferry, with water all around, according to a contractor who works for the parish. That ferry was the only way off that flooded spit of land. A source told ABC News that nearly the entirety of the area flooded, and winds howled at 35-40 mph, preventing a ferry from approaching.
It was estimated that it would be six to eight hours before it was safe for the ferry to motor out to the stranded people, who were without power but had cell phone service.
Thousands who live in the area were stuck in their homes or attics, and rescuers were out in boats helping those who needed it most.
There were 19 parishes included in the federal disaster declaration, while approximately 8,200 national guardsmen were available to help with search and rescue efforts, according to Gov. Jindal. There were 4,130 people in shelters across the state -- 730 in state-run shelters and 3,237 evacuees in parish run shelters, according to Jindal.
"I've got a four-by-four hole in my roof, several pieces in the front yard, the back wall of my house moved a couple of feet, and with each gust of wind, it's like you're breathing in and out," William Harold "Billy" Nungesser, president of Plaquemines Parish, told "Good Morning America."
Nungesser confirmed that a levee in Plaquemines Parish was overtopped with water, causing flooding. There were no reports that the $14 billion of levees and pumps put up around New Orleans after Katrina have been breached, but officials had not fully assessed the situation.
"The water came up so quickly and overtopped the levees from Breakaway to White Ditch on the east back of the north end of the parish. It's an area that we called for a mandatory evacuation," he said.
At the first crack of daylight today, parish officials were out examining the damage, according to James Madere, a parish geographic information system analyst.
In New Orleans, power lines were down, snaking and sparking across city streets after transformers exploded across the city Tuesday night.
The city saw handfuls of arrests as looters took advantage of the chaos. Police and the National Guard were all out in force.
The hurricane promised to lend even more solemnity to commemoration ceremonies Wednesday for Katrina's 1,800 dead in Louisiana and Mississippi, including the tolling of the bells at St. Louis Cathedral overlooking New Orleans' Jackson Square.
Isaac's top wind speeds rated it as a weak Category 1 storm, far less powerful than Katrina, which caused at least $81 billion in damage and was rated as the most powerful type of storm at Category 5.
At 200 miles wide, Isaac made initial landfall Tuesday evening before moving back into the Gulf of Mexico. Isaac's center remained over water, where it was almost stationary before making landfall again this morning.
The greatest concern was an expected storm surge of between six and 12 feet off the Louisiana and Mississippi coasts, four to eight feet along the Alabama coast and three to six feet on the Florida Panhandle, according to the Hurricane Center located in Miami.
A storm surge of eight feet was reported at Shell Beach, La. and in Waveland, Miss., according to the Hurricane Center late Tuesday.
In Mississippi, highway U.S. 90 was closed in sections by storm surge flooding. At one spot in Biloxi, a foot of water covered the in-town highway for a couple of blocks before high tide.
Tornado warnings swarmed the state throughout the morning as 55 mile per hour gusts hit the region.
The highest wind gust was recorded at 113 miles an hour overnight in Belle Chasse, Plaquemines Parish, La.
Thursday night into Saturday, Isaac is expected to move into the Mississippi Valley and, eventually, into Illinois and Indiana with possibly six inches of rain for the drought-stricken Midwest.
While traffic was nearly nonexistent Tuesday night, a few French Quarter bars remained open and filled with locals in New Orleans. At Lafitte's Blacksmith Shop -- the 150-year-old dive at the end of Bourbon Street -- Chris LaRue recommended the four staples of hurricane preparedness: "water, canned food, candles and booze."
"We're going to have some water to clean up," said LaRue. "But this kind of wind is nothing."
In advance of the storm, Louisiana set up shelters and stockpiled more than a million packaged meals, 1.4 million bottles of water and 17,000 tarps.
Since the levees failed in Katrina seven years ago, more than $14 billion has been spent on the 133 miles of floodwalls, spillways, gates and pumps surrounding New Orleans.
ABC News' Max Golembo and The Associated Press contributed to this report.