President Barack Obama declared federal emergencies in Louisiana and Mississippi late Wednesday, allowing federal aid to be freed up for affected areas.
Isaac arrived seven years after Hurricane Katrina and passed slightly to the west of New Orleans, where the city's fortified levee system easily handled the assault.
"Unfortunately, that's not been the case for low-lying areas outside the federal system, in particular lower Jefferson and Plaquemines parishes," said Louisiana Democratic U.S. Sen. Mary Landrieu. "Hurricane Isaac has reinforced for us once again just how vulnerable these critical areas are. We must re-engage the Corps of Engineers on this."
Louisiana's Public Service Commission said 901,000 homes and businesses around the state — about 47 percent of all customers — were without power Thursday. Utility company Entergy said that included about 157,000 in New Orleans.
New Orleans' biggest problems seemed to be downed power lines, scattered tree limbs and minor flooding.
"Hopefully, as far as the city of New Orleans is concerned, the worst is behind us," Mayor Mitch Landrieu said Thursday.
In Plaquemines Parish, a sparsely populated area south of the city that is outside the federal levee system, dozens of people were stranded in flooded coastal areas and had to be rescued. The storm pushed water over an 18-mile levee and put so much pressure on it that authorities planned to intentionally puncture the floodwall Thursday to relieve the strain.
Officials rushed to evacuate more than 100 nursing home residents Wednesday from Plaquemines Parish, an area with a reputation for residents hunkering down to weather storms and perhaps the hardest hit by Isaac. In this hardscrabble, mostly rural parish, even the sick and elderly are hardened storm veterans.
"I don't think we had to evacuate to begin with," said Romaine Dahl, 59, as he sat in a wheelchair. "The weather was a hell of a lot worse last night than it is now. And I got an idea that after all this is said and done they're going to say everything is over with, go on back home."
By midafternoon Wednesday, Isaac had been downgraded to a tropical storm. The Louisiana National Guard ceased rescue operations in Plaquemines Parish, saying it felt confident it had gotten everyone out. There were no serious injuries. National Guard spokesman Capt. Lance Cagnolatti said guardsmen would stay in the area over the coming days to help.
Isaac's maximum sustained winds had decreased to 40 mph, and the National Hurricane Center said it was expected to become a tropical depression by Thursday night, meaning its top sustained winds would drop below 39 mph. The storm's center was on track to cross Arkansas on Friday and southern Missouri on Friday night, spreading rain as it goes.
Forecasters expected Isaac to move farther inland over the next several days, dumping rain on drought-stricken states across the nation's midsection before finally breaking up over the weekend.
Isaac came ashore late Tuesday as a Category 1 hurricane, with 80 mph winds near the mouth of the Mississippi River. It drove a wall of water nearly 11 feet high inland.
Contributing to this report were Associated Press writers Brian Schwaner and Stacy Plaisance in New Orleans; Melinda Deslatte in Baton Rouge; Kevin McGill in Houma; Vicki Smith in LaPlace; Kevin McGill in Houma; Holbrook Mohr in Waveland and Pass Christian, Miss.; and Jeff Amy in Biloxi and Gulfport, Miss.
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