Updated at 5:06 a.m. ET Wednesday: The center of Hurricane Isaac made its second landfall in southeastern Louisiana early Wednesday, officials said.
The storm hit just west of Port Fourchon, La., with maximum sustained winds of 80 mph at around 2:15 a.m. local time (3:15 a.m. ET), according to aircraft and radar data from the National Hurricane Center.
Emergency management officials in Plaquemines Parish, southeast of New Orleans, reported overtopping on a levee from Braithwaite to White Ditch early on Wednesday. "This will result in significant deep flooding in this area," the National Weather Service said.
Earlier, Isaac produced a dangerous storm surge along the northern Gulf coast after wobbling back out to sea two hours after its initial landfall on Tuesday night. Flooding from rainfall was expected, the center said.
The storm surge combined with a high tide will cause normally dry areas near the Mississippi and southeastern Louisiana coast to be flooded by peaks of 6 to 12 feet, the center said. Alabama could see up to 8 feet; the Florida panhandle, 6 feet.
A 10-foot surge was reported at Shell Beach, La., the center said.
Forecasters predicted the storm would arrive in New Orleans early Wednesday and then head for Baton Rouge.
While not packing nearly the power of Katrina -- which was a Category 3 storm when it slammed New Orleans on August 29, 2005 -- Category 1 Isaac was nevertheless a powerful reminder of New Orleans' vulnerability.
'Really bad weather'
The hurricane will be the first test for multibillion-dollar flood defenses built after levees failed under Katrina's storm surge and left large parts of New Orleans under water.
The hurricane center continued to warn that flooding from rainfall and storm surge remains the storm’s greatest threat. The slow-moving storm is expected to dump up to 20 inches of rain in some spots over two days.
Hurricane Isaac initially made landfall at 8 p.m. Tuesday in southeastern Louisiana as a Category 1 hurricane.
"It's going to be a long period of really bad weather" for the Louisiana and Mississippi coasts as well as areas inland, National Hurricane Center Director Rick Knabb said. Even before landfall, some flooded roads and power outages were reported in those states.
New Orleans Mayor Mitch Landrieu said he expects his city "will get the brunt of it." Nola.com reported. Entergy New Orleans, the power company that supplies the region, reported outages for more than 300,000 customers.
"We think that we're well prepared," Landrieu said at a briefing, while emphasizing that much depends on how well residents heed warnings to hunker down.
No mandatory evacuations were ordered inside New Orleans, which sits behind levees and pumps reinforced after Hurricane Katrina.
The sewer system in one lakefront community, Northshore Beach, in St. Tammany Parish had to be shut down because floodwaters rose over sewage lift stations, emergency management officials said.
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While Isaac is well below the intensity of Katrina, its vast size and slow track have forecasters predicting widespread flooding.
Hundreds of Army National Guard troops took up positions around New Orleans to ward off any threat of looting.
One man was arrested in Lafourche Parish on Tuesday night after reports he broke into a vehicle and then attempted to break into a house, WDSU reported.
Sheriff Craig Webre described the alleged act as "a heinous example of someone who truly has no regard for the rights of law-abiding citizens."
The guard's arrival came as bands of driving rain and stiff winds began battering the city and other parts of the coast. Some 10,000 homes and businesses had lost power in southern Louisiana by late afternoon, as did 6,000 customers in Mobile, Ala.
President Barack Obama added his voice to those of local officials urging residents to hunker down or evacuate if told to do so. "Now's not the time to tempt fate," he said in brief comments Tuesday morning. "Listen to your local officials and follow their directions, including if they tell you to evacuate."
"The inland flooding from the heavy rainfall could extend hundreds of miles from the coast," Knabb said.
Isaac is wide as storms go, with tropical storm-force winds stretching 185 miles from its center.
By Tuesday afternoon, some beach areas were seeing water lapping onto streets.
Rainfall of 7 to 14 inches across the coast as well as inland is likely, and a few places could even see 20 inches, Knabb said.
Residents should expect "a lot of hazards to contend with, even isolated tornadoes" into Wednesday, Knabb said.
Isaac was expected to arrive in New Orleans seven years to the day Katrina struck on Aug. 29, 2005, killing more than 1,800 people and causing billions of dollars of damage. Levees built or repaired after Katrina are designed to withstand far more than that 12-foot surge, in some cases storm surges as high as 26 feet.
Mandatory evacuations were issued Monday for unprotected, low-lying areas outside New Orleans, as well as low-lying areas in Mississippi.
Residents in coastal communities from Louisiana to Mississippi stocked up on food and water and tried to secure their homes, cars and boats.
"Right now we’re starting to experience some flooding of low-lying areas along the beachfront," Brian Adam, emergency management director in Mississippi's Hancock County, told NBC News. "We’ve opened two shelters and have about 185 people there."
In Bay St. Louis, Miss., residents in low-lying areas evacuated while those on high ground were keeping an eye on Isaac, resident Ellis Anderson told NBC News.
From weather.com: Live updates and analysis
"It's not expected to be another Katrina," she said. "But everybody is watching it very seriously" because of the potential path that could push water into the area hard hit by Katrina and Hurricane Gustav in 2008.
Gustav "went to the west of New Orleans," she recalled, pushing "all that water into that cup that is the Gulf Coast of Mississippi."
In New Orleans, a bumper-to-bumper stream of vehicles left the city Monday on a highway toward Baton Rouge in search of higher ground. Others prepared or were forced to ride the storm out.
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Along Canal Street in New Orleans' historic French Quarter, crews boarded up the windows of some stores and businesses.
Offshore in the Gulf, regulators said that 93 percent of daily oil and 67 percent of daily natural gas production in U.S.-regulated areas have been shut down by the hurricane.
Isaac has killed at least 22 people and caused significant flooding and damage in Haiti and the Dominican Republic before skirting the southern tip of Florida on Sunday.
In the Atlantic Ocean, Tropical Storm Kirk formed about 1,230 miles northeast of of the Northern Leeward Islands and was moving west about 12 mph, the hurricane center reported. There was no immediate threat to land.
Reuters and The Associated Press contributed to this report.
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