Around 13 million people were battling "catastrophic" flooding and torrential rains in the storm-ravaged Texas where Hurricane Harvey has wreaked havoc, turning streets into raging rivers and claiming at least five lives.
Meteorologists have forecast that the historic rainfall will dump up to 50 inches by Wednesday.
Heavy rain bands were expected to move in over the Houston area overnight, which will continue the catastrophic and life-threatening flash-flood emergency in the area.
"A flood of this magnitude is an 800-year event, and it exceeds the design specification of our levees," Fort Bend County Judge Robert Hebert said in a statement today.
The flooding comes after Harvey, the most powerful hurricane to hit the US mainland in 13 years, left at least five people dead after it bashed the Texas Gulf coast.
Rescuers received more than 1,000 distress calls from people forced on their rooftops in the submerged Houston area.
Officials warned the danger was far from over. They said the flooding in Texas was unlikely to recede quickly and that the storm willforce more than 30,000 people from their homes.
Federal officials have widened the emergency zone to Louisiana, the southeastern US state on the Gulf of Mexico.
US President Donald Trump will travel to Texas tomorrow to get a first-hand information on the flooding.
Harvey has left a trail of destruction as it swept through Texas, pummelling the region with heavy rains and flash floods.
According to the Indian Consulate in Houston, at least 200 Indian students at University of Houston have been stranded due to the flooding. Houston is home to a sizable Indian population.
The hurricane has now turned into a tropical storm and is dumping rain on southern Texas.
The National Weather Service (NWS) called the flooding in Texas "unprecedented". "We are seeing catastrophic flooding, and thiswill likely expand and it will likely persist as it's slow to recede," its director Louis W Uccellini said today.
The immediate focus for authorities remained Houston, the fourth-largest city in the US and asprawling metropolitan area. Two reservoirs were opened to release water to relieve the stress the downpour has caused in the region, which has seen as much rain in a few days as it averages in an entire year.
Parts of Harris County, which encompasses Houston, were pelted with 30 inches of rain in 72 hours, the NWS said.
Harvey has been hammering Texas since Friday, when it first made landfall as a Category 4 hurricane. At least five people have died and the death toll is expected to rise as the storm has triggering record floods and tidal surges.
The last Category 4 storm to hit the US was Hurricane Charley in August 2004 in Florida.
Officials say they have conducted more than 250 water rescues and rescued more than 1,000 people.
Major refineries were closing down, cutting at least one million barrels per day out of production, and the Houston Ship Channel, the busiest in the nation, is shut. Officials said Houston, a major centre for the nation's energy industry, suffered billions of dollars in damage and would take years to fully recover.
Authorities have closed Houston schools for the week and shuttered William P Hobby Airport until at least the middle of the week.
Southwest Airlines has airlifted around 500 of its customers out of Hobby Airport, according to the airliner and the US government sources with knowledge of the operation.
"It's scary. I've been through many storms in my city, but it has never been this terrible. This city is completely underwater," Guatzin, a local resident, told CNN.
"We have nowhere to go. I have a 2016 Chevy Silverado and the water is up to the door panels. The water keeps rising," said Jake Lewis, who was in Houston on business and woke up to ankle-deep water in his Houston hotel room.
Janet Castillo, who was trapped at home with children, said: "The water keeps rising. We have called several numbers but no luck."
Rescuers in fishing boats, huge dump trucks and even front-end loaders battled rains to move people to shelter. Some used inflatable toys to ferry their families out of inundated neighbourhoods, wading through chest-deep water on foot while the region was under near-constant tornado watches.
The Brazos River, which runs southwest of Houston, is expected to reach record heights in the coming days.
Over the next few days, Harvey is forecast to head back into the Gulf of Mexico, where it will pick up moisture before moving back over Galveston and into Houston again, CNN meteorologist Karen Maginnis said, meaning at least four more days of rainfall.
Harvey is the first major natural disaster to strike the US under the Trump presidency. Former president George W Bush was heavily criticised for the federal government's slow response to Hurricane Katrina in New Orleans in August 2005.
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