NEW ORLEANS — Hurricane Isaac hovered over the Gulf Coast on Wednesday morning, punishing southeast Louisiana with 80 mile per hour wind gusts, driving, horizontal rain and the threat of calamitous flooding. Forecasters said the rainfall may not let up for days.


The hurricane, which made its second landfall early Wednesday, was essentially stationary just off the Louisiana shore, according to the National Weather Service, bringing with it the heightened risk of tornadoes and flash flooding hundreds of miles inland from Louisiana, and across Mississippi and Alabama to Florida.


The longer the storm lingers, the more pressure it is putting on the levees and other flood-protection systems along the coast.


In Plaquemines Parish, about 95 miles from New Orleans and where the hurricane first made landfall, water “overtopped” a levee, causing extensive flooding, according to the National Weather Service.


The levee is not one of the large, federally maintained earthworks lining the Mississippi River, but a locally maintained levee some 8 feet high, and lower than the 12-foot surge that hit it, according to officials from the Army Corps of Engineers. The water is threatening people living along the east bank of the parish, near the mouth of the Mississippi River, who did not comply with the mandatory evacuation.


“Right now, we’re trying to figure out where they actually are,” said a state trooper, Russel Brueck, at the parish emergency operations center. “They’re not actually in the body of water. They’re just stuck.”


In the early morning, as the hurricane stalked up into the bayous of southern Louisiana, fierce wind and pounding rain arrived in earnest here in New Orleans. Wind gales howled through the streets, blowing apart billboards, tugging down trees and flooding streets.


More than 520,000 residents of Louisiana were without power, nearly a third of them in New Orleans. They had nothing to do but wait, since Entergy, the utility, could not send workers to fix lines until winds were below 30 miles per hour.


As they awaited daylight, emergency officials across south Louisiana braced for another long day.


In Jefferson Parish, there was anxiety about two communities not protected by levees — Grand Isle and Lafitte.


Mandatory evacuations had been ordered in both places, but while only about 30 residents remained in Grand Isle as Hurricane Isaac bore down on them, far fewer had heeded the warning in Lafitte.


“Initially, the storm only being a tropical storm instead of a hurricane, many people, especially the people who live down there, didn’t have a whole lot of concern,” said Deano Bonano, an aide to a parish councilman. “Then it ramped up pretty quickly.”


By 11 p.m. Tuesday night, two feet of water had inundated parts of Grand Isle, a barrier island. There was significantly less flooding in Lafitte. Though the storm’s direction had helped by pushing water out, that direction may change later Wednesday, and flooding there was likely.


Hurricane Isaac is the first test of a $14.5 billion, 133-mile ring of levees, flood walls, gates and pumps installed after Hurricane Katrina by the Army Corps of Engineers, the agency that built the defenses that failed New Orleans catastrophically in 2005.


While the current storm is nowhere near as powerful as Hurricane Katrina — which struck seven years ago Wednesday — its pounding, driving rains and surging waves are lashing towns from east of Morgan City, La., to the Mississippi-Alabama border.


Waters from the gulf pushed onto the coast through the night in Mississippi and Alabama, where thousands had lost power and 60 m.p.h. winds knocked out transformers and stripped palm trees. Several inches of rain had fallen overnight, flooding parts of the small cities along the coast. The Mississippi Gaming Commission ordered the 12 casinos along the coast to close.