HOUSTON, Aug 28 (Agencies): Swollen rivers in east Texas aren't expected to crest until later this week, but federal officials are already predicting Harvey will drive 30,000 people into shelters and spur 450,000 to seek some sort of disaster assistance.
Several locales have already received two feet or more of rain, and forecasters say a reprieve won't arrive till week's end at the earliest.
"This is a landmark event for Texas," said FEMA Administrator Brock Long. "Texas has never seen an event like this."
The US city of Houston is in the grip of the biggest storm in the history of the state of Texas, officials say.
A record 30in of rain (75cm) has fallen on the city in the wake of Hurricane Harvey, turning roads into rivers.
The area is expected to have received a year's rainfall within a week. Five people are reported dead. Helicopters have plucked victims from rooftops.
With rescue services overstretched as the rain continues, many people are having to fend for themselves.
Harvey made landfall as a category-four hurricane late on Friday. It was later downgraded to a tropical storm.
Up to 2,000 people have been rescued in and around Houston, the fourth-largest city in the US, where about 6.6m people live in the metropolitan area.
An inundated care home in Dickinson, about 30 miles (50 km) south east of the city, has now been evacuated by helicopter after an image of several elderly women sitting in a lounge in waist-deep water went viral on social media.
The US National Weather Service (NWS) says conditions are "unprecedented". A "flash flood emergency" is in force across the Houston area, with travel near impossible.
Thousands of homes are without electricity. Many schools are closed - as are the two main airports, with runways completely flooded.
BBC graphic showings levels of heavy rain fall and associated risks. 13 inches over 24 hours is equal to severe flooding.
Entire suburbs are under water, shops and businesses are shut and, with the motorways around the city cut off and both airports closed, travel is all but impossible.
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