Texans are being warned to brace themselves for "unprecedented" rainfall of up to 1.3 metres, as record floods, tidal surges and tornadoes hit communities already devastated by Tropical Storm Harvey.
- At least two dead from the storm
- Harvey now about 170 kilometres from Houston
- Average yearly rainfall for Texas expected during storm
- 3,000 National Guard members called in, Donald Trump to visit on Tuesday
Emergency crews raced to pull people from cars and homes as flood waters rose on Sunday, rescuing more than 1,000 people around Houston, a city of about 2 million — the fourth most populous in the United States.
At least two people are dead but the death toll is expected to rise as the storm triggers additional tidal surges and tornadoes, with parts of the region expected to see a year's worth of rainfall in the span of a week.
"This event is unprecedented and all impacts are unknown and beyond anything experienced," the National Weather Service said on Twitter.
The White House said US President Donald Trump would travel to the area on Tuesday.
Mr Trump, facing the first big US natural disaster since he took office in January, signed a disaster proclamation on Friday, triggering federal relief efforts.
"We are dealing with a triangle of ongoing rain, including Houston, Victoria and Corpus Christi cities. We will continue receiving incredibly heavy rain," Texas Governor Greg Abbott said.
He said 250 highways had been closed around Texas and 3,000 National Guard members had been called in to help.
Mr Abbott said the number of counties declared federal disaster areas from the storm and its aftermath had increased to 18, covering nearly a quarter of Texas's population.
As the water rose, the National Weather Service offered another ominous forecast: before the storm passes, some parts of Houston and its suburbs could receive as much as 1.3 metres of rain.
"The breadth and intensity of this rainfall is beyond anything experienced before," the National Weather Service said in a statement.
The director of the Federal Emergency Management Agency (FEMA), Brock Long, said the aftermath of the storm would require FEMA's involvement for years.
"This disaster's going to be a landmark event," Mr Long said.
The centre of Harvey was still 200 kilometres away from Houston, and was forecast to arc slowly toward the city until Wednesday.
Forecasters said Harvey was on track to go back into the Gulf of Mexico for some slight strengthening before returning into Texas again.
The storm struck at the heart of the country's oil and gas industry, forcing operators to close several refineries and evacuate and close offshore platforms.
People urged to get onto rooftops
The storm turned roads into rivers and caused chest-deep flooding on some streets in Houston as rivers and channels overflowed their banks.
The floods forced thousands of people to rooftops or higher ground and overwhelmed rescuers who could not keep up with the constant calls for help.
Floodwaters rose swiftly in Houston and emergency services told the city's 2.3 million inhabitants to climb onto the roofs of houses, if necessary.
In a rescue effort that recalled the aftermath of Hurricane Katrina, helicopters landed near flooded freeways, airboats buzzed across inundated neighbourhoods and high-clearance vehicles ploughed through water-logged intersections.
Volunteers joined emergency teams to pull people from their homes or from the water, which was high enough in some places to gush into second floors.
The flooding was so widespread that authorities had trouble pinpointing the worst areas.
They urged people to get on top of their homes to avoid becoming trapped in attics and to wave sheets or towels to draw attention to their location.
Houston mayor Sylvester Turner said police and fire departments had received nearly 6,000 calls for rescues.
"I don't need to tell anyone this is a very, very serious and unprecedented storm," Mr Turner told a news conference.
"We have several hundred structural flooding reports. We expect that number to rise pretty dramatically."
Strongest Texas storm in more than 50 years
Hundreds of people were sheltering at the downtown George Brown Convention Centre, where they sought water, food and baby supplies. Some people were being brought to the centre in city garbage trucks.
The Harris County Sheriff's Office rescued more than 2,000 people in the greater Houston area using vehicles including motorboats, airboats and Humvees on Sunday, a spokesman said.
The Coast Guard, which received more than 300 requests for help, deployed five helicopters and asked for additional aircraft from New Orleans.
On Friday night, a man died in a house fire in the town of Rockport, 48 kilometres north of Corpus Christi.
The second confirmed fatality came on Saturday when an elderly woman drowned attempting to drive through flooded streets in west Houston, police said.
Staff at a Houston television station broadcasting live coverage of the floods had to evacuate after water started to gush into the building.
The anchors and news operations at KHOU moved first to a second floor before finally abandoning the station.
The fiercest hurricane to hit the US in more than a decade came ashore late Friday (local time) about 50 kilometres north-east of Corpus Christi as a mammoth Category 4 storm with 209 kilometre per hour winds.
The system was the strongest to strike Texas since 1961's Hurricane Carla, the most powerful Texas hurricane on record.