Weather forecasters have downgraded Hurricane Isaac to a tropical storm, but warn the weather system is still a major threat to life and property for residents along the U.S. Gulf Coast.

Isaac's top sustained winds have dropped to 110 kilometers per hour. But the storm is still pounding Louisiana and Mississippi with massive rainfall, floods, and isolated tornadoes.

Engineers in the city of New Orleans are considering blowing a hole in a levee to allow flood waters to flow back into the Mississippi River.

New Orleans authorities have imposed a dusk-to-dawn curfew. More than half a million people have no electricity and emergency workers have had to rescue people trapped in cars and stuck on rooftops. They are now waiting for the wind and rain to subside before searching for more storm victims and fully assessing the damage.

Forecasters say Tropical Storm Isaac will crawl across Louisiana Thursday before moving into the neighboring state of Arkansas Friday and into the drought-stricken midwestern U.S.

Isaac's rainfall could total 50 centimeters in some places, and parts of Louisiana and Mississippi could see storm surges as high as four meters.

U.S. President Barack Obama declared states of emergency for 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.

Isaac came ashore with just a fraction of the destructive power of Hurricane Katrina, which slammed into New Orleans seven years ago Wednesday, killing 1,800 and crippling the city. Experts also credit the fortified levees and storm defenses built after Katrina.