U.S. weather forecasters have downgraded Tropical Storm Isaac to a depression but warn the danger to the southern United States from the system is not over.

The National Hurricane Center says the slow-moving Isaac weakened over northern Louisiana Thursday. The system is expected to produce more rain as it moves toward the north. Forecasters say isolated parts of Louisiana, Mississippi, Alabama and Arkansas could get as much as 63 centimeters of rain through Friday.

The hurricane center warned there could still be more flooding across southeastern Louisiana and Mississippi and that tornadoes are possible.

Forecasters are now keeping a close watch on another system, Kirk, which is now the fifth-named storm of this year's Atlantic hurricane season.

Kirk is a Category One hurricane with maximum sustained winds of 150 kilometers per hour. Forecasters say it is rapidly moving west over the Atlantic Ocean and is likely to strengthen into a Category Two storm on Friday.

Isaac struck Louisiana and the neighboring Gulf Coast states of Mississippi and Alabama as a minimal, Category One hurricane, causing strong winds, heavy rain, and massive flooding. Hundreds of residents in Louisiana's Plaquemines Parish, a small coastal area located south of New Orleans, were trapped after floodwaters spilled over levees.

The parish's president, Billy Nungesser, said the flooding in his community was unprecedented, surpassing the damage from deadly Hurricane Katrina in 2005.

''We've never seen anything like this, not even Katrina.''

Authorities and residents scrambled to rescue neighbors who had ignored evacuation orders ahead of Isaac's arrival.

The storm hit the U.S. coastline seven years to the day that Katrina devastated New Orleans and killed 1,800 people in the region. Isaac caused more than 20 deaths on Caribbean islands, but only one possible storm-related death was reported in the United States.

Federal officials say the extensive system of levees, canals and pumps built after Katrina helped spare New Orleans from extensive damage this time.

Seven-hundred-thousand people were without power at the peak of the storm. Authorities say they will assess storm damage and look for victims once the wind and rain let up.

U.S. President Barack Obama declared emergencies in Louisiana and Mississippi, making both states eligible for federal funding and other aid. Emergency assistance teams were dispatched to the region in advance of the storm.