(UNDATED) -Hurricane Isaac made its second landfall west of Port Fourchon in southeast Louisiana, the National Hurricane Center said.
The storm made its initial landfall Tuesday evening along the mouth of the Mississippi River.
The storm is moving, but slowly, northwesterly at 8 miles per hour.
Heavy rains, storm surges and strong winds are expected to continue for several hours.
The strong winds and pounding rain are generating the first real test early Wednesday of flood control systems and emergency services in New Orleans on the seventh anniversary of Hurricane Katrina.
With forecasters warning Isaac was generating dangerous storm surges and flooding from heavy rain, all eyes were on the New Orleans levee system that was rebuilt and reinforced at a cost of $14 billion after it failed when Katrina struck in 2005.
Nearly 1,800 people died in Katrina, the majority when levees failed and flooded New Orleans.
Issac made its second landfall at about 2 a.m. CT (3 a.m. ET) near Port Fourchon, 60 miles southeast of New Orleans, in southeast Louisiana 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 updated.
After stalling along the coast, the hurricane was moving at about 8 mph -- with sustained winds of 80 mph, the hurricane center said.
Even with more dangerous conditions likely yet to come, the storm already has caused significant surges and flooding in a number of locales, and not just those directly in Isaac's path. Storm surges of 9.9 feet have been reported in Shell Beach, Louisiana, and 6.2 feet in Waveland, Mississippi, according to the hurricane center.
These surges likely will get worse, with forecasters predicting water levels to rise between 6 to 12 feet on the coast in Mississippi and southeastern Louisiana alone.
The National Weather Service, meanwhile, warned early Wednesday that heavy rainfall across metropolitan New Orleans and nearby coastal communities will likely result in flash floods.
The worst of the storm was expected to hit New Orleans in the early morning hours Wednesday, though the first to feel its fury was Plaquemines Parish where Isaac first made landfall Tuesday night before moving back out over the water toward the mouth of the Mississippi River.
Emergency management officials reported the overtopping of an 18-mile stretch of a levee in Plaquemines Parish from Braithwaite to White Ditch, which will "result in significant deep flooding in the area," the National Weather Service of New Orleans said in a statement early Wednesday.
Strong winds ripped through the parish, knocking down utility and power poles.
Isaac, which started as 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 was classified as a Category 1 hurricane. It is significantly weaker than Category 3 Katrina, though forecasters warn it is capable of causing significant flooding.
Power had been knocked out to more than 310,000 customers in Louisiana by the storm, while localized street flooding and downed electrical wires were reported across the city, according to officials. A handful of residents also were evacuated after trees uprooted by wind smashed into several homes, the city said.
Isaac earlier prompted three airports to close - in New Orleans; Gulfport-Biloxi, Mississippi; and Mobile, Alabama - and cancellations of around 1,500 flights, according to airline and airport officials.
Major ports along the Gulf Coast and the Mississippi River from Baton Rouge to its mouth have been closed, according to the Coast Guard.
In Mississippi, more than 1,800 people were staying in 33 shelters located in 16 counties, according to the state's emergency management agency.
Shrimpers in Bayou Le Batre, Alabama, were among those who heeded official warnings and hunkered down.
The Nuclear Regulatory Commission said it was sending additional inspectors to two Louisiana nuclear plants in the storm's path, as power company Entergy planned a "controlled shutdown" of one of them starting Tuesday afternoon.
Wholesale gas prices declined Tuesday after surging nearly 8 cents a day earlier as then-tropical storm Isaac veered toward the Gulf Coast, causing many oil refineries to shut down as a precaution.
Analysts said panic buying was over and that Isaac was unlikely to do lasting damage to oil refineries and infrastructure along the Gulf Coast.
CNN's Ed Lavandera, Anika Chin, Mike Ahlers, Aaron Cooper and Ed Payne contributed to this report.
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