JUDY WOODRUFF: Hurricane Isaac weakened today to a tropical storm, but that was little comfort to the people of Louisiana. The system spent a long day battering the state, after moving ashore last night. It forced a curfew in New Orleans and new evacuations outside the city.
RAY SUAREZ: Hour after hour, the storm slowly plodded inland. Heavy winds and lashing rains radiated hundreds of miles from the eye, and up to 20 inches of rain was forecast in some places. Louisiana Governor Bobby Jindal issued a new warning this afternoon in Baton Rouge.
GOV. BOBBY JINDAL (R), Louisiana: We ask people to use their common sense, exercise caution. If you are somewhere in an impacted parish where you are safe, we recommend you stay there. If you do not need to, do not travel on these roads, especially with these gusts, these strong winds, as well as the chance for localized flooding.
RAY SUAREZ: The storm pushed massive amounts of water into lowlands of the central Gulf Coast, with surges up to 15 feet across the Louisiana and Mississippi coastlines. The full force began arriving last night, but it was felt most today, the seventh anniversary of Hurricane Katrina's landfall.
In New Orleans, some Katrina veterans said the far weaker Isaac still packed a wallop.
GARY KING, Progressive Guitar Shop: This is the second time we get hit really hard, because the first time with Katrina we lost everything. My instruments were actually floating down the road. We have hunkered down from last night and we're just hoping for the best.
RAY SUAREZ: There were scattered reports of looting, prompting Mayor Mitch Landrieu to impose the overnight curfew. But the city seemed the avoid the worst, as the $14.5 billion levee system, rebuilt after Katrina, held its own.
MAJ. GEN. JOHN PEABODY, Army Corps of Engineers: The Corps has been preparing for an event like this or almost like this for seven years. And we're finding that the system that we built, the hurricane storm damage reduction system in New Orleans, is functioning as designed.
All the major closures have been executed. However, we have a lot of folks that are not inside New Orleans or inside that system that are at risk.
RAY SUAREZ: And south of the city in Plaquemines Parish, a levee outside that system was overtopped by the surge, flooding the area. Boats rescued flooded residents of the town of Braithwaite just 10 miles from downtown New Orleans.
WOMAN: It is horrible. Everybody's house is gone. Nobody got a house in Braithwaite, nobody.
MAN: How high is the water?
WOMAN: The water is almost over my head.
RAY SUAREZ: Parish President Billy Nungesser lost part of the roof of his home overnight. He spoke with ABC this morning.
BILLY NUNGESSER, President, Plaquemines Parish, Louisiana: I think my house is going to have more damage from this storm than Katrina. This is not a Category 1. This storm has been relentless. And the wind and rain has not given up, has not slowed up in this parish for not a minute since this whole thing started.
RAY SUAREZ: But in St. Tammany Parish, across Lake Pontchartrain from New Orleans, officials said it appeared the worst had come and gone.
MARTY GOULD, Council Chairman, St. Tammany Parish, Louisiana: We're hopeful that the northwest winds are going to continue to help us. We're seeing signs along the Gulf Coast where the water levels are receding, which is a good thing, because that's where our water ends up coming from.
RAY SUAREZ: In all, nearly 10,000 Louisianians spent the day in shelters. Hundreds of thousands more lost power and faced the prospect of going days in the dark, as the storm and its after-effects linger. Some had planned ahead.
MAN: When power goes out, it's always a pain, but we lucked out, and had a generator.
RAY SUAREZ: Meanwhile, in Mississippi, Isaac piled up water that drowned major roads along the coast.
Governor Phil Bryant surveyed the situation in Gulfport.
GOV. PHIL BRYANT (R), Mississippi: We have got to surge that has covered Highway 90 and so that's a problem. Highway 603 is also covered. So if you're in Waveland, Miss., for example, the two routes out of that city are now blocked by water.
RAY SUAREZ: And with the storm in no hurry, that water could keep coming for some time.
For more on the slow progress of Isaac through the Gulf Coast, I am joined by Rick Knabb, director of the National Hurricane Center. Rick, welcome to the "NewsHour."
RICK KNABB, Director, National Hurricane Center: Thank you for having me.
RAY SUAREZ: We can see from that graphic over your shoulder a storm that isn't moving quickly. What makes a storm like Isaac stop and pretty much stay in one place?
RICK KNABB: The forward motion of a tropical storm or hurricane is determined by its surrounding steering currents, essentially a cork in a stream analogy.
It doesn't steer itself. It has the surrounding environment to push it around, and it has found a weakness in which the atmospheric steering currents are not all that strong. It's moving very slowly. And a slow-moving, large storm like Isaac is poses serious water-related hazards. A bigger storm much more capable of producing storm surge flooding from the Gulf of Mexico over land areas and coastal regions and a large slow mover is going to dump a lot of rain and cause inland flooding near the coast and way inland for a few days to come.
RAY SUAREZ: People are talking about the comparison to Hurricane Katrina, which had much, much faster top winds, and they're talking about Isaac being just as destructive. How is that?
RICK KNABB: It depends on where you are.
In some places, you might receive greater impacts from this storm than you did during Katrina. In other places, it will not be nearly as bad. And that's because it's very localized. And every storm is very different. This is not taking the same exact track that Katrina did. It's going in west of the mouth of the Mississippi River.
So, for example, down in Plaquemines Parish, south of New Orleans, they have been getting this onshore flow out of the southwest. That was a different configuration than when Katrina came through and all the winds were coming in from this direction, just a totally different situation for any local spot.
So, some folks are going to see greater impacts this time, and that's one thing that often happens. We try to gauge our expectation for the current storm based on our experience with the past one. And, locally, that doesn't often apply.
RAY SUAREZ: What do the next 24 to 72 hours hold? Can this thing slowly die out, run out of supplies of moisture, stop dumping all this rain?
RICK KNABB: Slowly, it's going to run out of its energy, but slowly is the operative word here because it's a big storm.
Big ones take a lot longer to spin down, and this is bringing so much moisture into the South Central United States. It's going to take well into tomorrow morning for the winds and the rains to calm down on the coastal part of the northern Gulf here. But then there are other places where it hasn't even started yet, and they are in for a day-and-a-half to two days of heavy rains and flash flooding, and then eventually, longer term river flooding could materialize in states that haven't even started to see the rain yet.
This is another lesson for us to learn that they're not just coastal events. So people farther north that don't think they have a tropical storm issue where they live, this one could bring heavy rains. And we have lost too much lives in the past from inland flooding, people getting in the cars and dying in flooded roadways.
RAY SUAREZ: Rick Knabb of the National Hurricane Center, thanks for joining us.
RICK KNABB: Thank you for having me.