The Visible Infrared Imaging Radiometer Suite (VIIRS) on the Suomi-NPP satellite captured this night time view of Hurricane Isaac moving over the Gulf Coast of the US on Aug 29, 2012 in this NASA handout image. The slow-moving but powerful Category 1 hurricane was felt along the Gulf Coast, threatening to flood towns in Mississippi and Louisiana with storm surges of up to 12 feet (3.7 meters) and top sustained winds up to 75 miles per hour (120 kilometers per hour). [Photo/Agencies]

WASHINGTON - Hurricane Isaac weakened to a tropical storm on Wednesday afternoon with storm surge and inland flooding still threatening, said the US National Hurricane Center on the 7th anniversary of Hurricane Katrina's destructive arrival.

The US National Hurricane Center said Tropical Storm Isaac is currently moving on at 6 mph (9.7 kph), with the maximum sustained wind of 70mph (113 kph), and centered about 50 miles (80.5 kilometers) southwest of New Orleans, Louisiana.

But the center warned life-threatening hazards from storm surge and inland flooding still remained.

Although Isaac was much weaker than Katrina, forecasters said the current slow movement and the large size of Isaac could cause hours longer impact on residents living along its path.

Isaac has lashed heavy rains and winds to the state of Louisiana since it made landfall in southeastern US state late Tuesday as a Category One Hurricane.

Early Wednesday, a storm surge has topped a levee in Plaquemines Parish south of New Orleans, the largest city of Louisiana and most-devastated area seven years ago by Hurricane Katrina.

Local officials said they had been working with US Coast Guard to rescue people stranded on top of the levee, and they may have to breach the levee in the flooded area.

Utility companies said more than 500,000 people were left without power across Alabama, Louisiana and Mississippi.

The weather condition will persist through Wednesday night or early Thursday, said Richard Knabb, director of the National Hurricane Center.

Dozens of families that had ignored mandatory evacuation orders in a low-lying area had to retreat to roofs and sought rescue amid fierce wind and slashing rains, the CNN reported.

The southern US states of Florida, Louisiana, Mississippi and Alabama have been on high alert against the threat of Isaac. Thousands of residents living in Louisiana, Mississippi and Alabama were told to leave their coastal houses since Monday.

Earlier Wednesday, US President Barack Obama called Governors of Alabama, Mississippi and Louisiana as well as Mayor of New Orleans, pledging that the federal government was ready to provide local officials with necessary support and resources needed. He also acknowledged the 7th anniversary of Hurricane Katrina provides of the lasting effects these types of storms can have on communities.

Wednesday marks the 7th anniversary of Hurricane Katrina, which devastated New Orleans and the Gulf Coast on August 29, 2005. The hurricane was the costliest natural disaster, and one of the five deadliest hurricanes, in the history of the United States. At least 1,836 people died and total property damage was estimated at $81 billion.

A day earlier, Obama warned residents living alongside the expected path of Isaac of significant flooding and other damages. He urged them to evacuate in time to safe grounds.

Also, Florida, Louisiana and Alabama have already declared states of emergency over the weekend, fearing that Isaac might repeat the disaster that Hurricane Katrina brought to the Gulf coast just seven years ago.