Hurricane Harvey on Friday further intensified into a dangerous category four storm, just hours before it was due to slam into Texas with a force not felt on the US mainland since 2005.
Texas Governor Greg Abbott asked President Donald Trump to declare Harvey a "major disaster" in order to speed federal assistance, issuing disaster declarations for 30 counties.
The arrival of the storm — which was packing sustained winds of 130 miles (215 kilometers) an hour — was the first major domestic challenge for Trump, who the White House said would head to the affected region early next week.
"We can obviously tell already at this stage this is going to be a very major disaster," a somber Abbott said, as more than 1,000 National Guardsmen were activated to help with evacuation and recovery.
"We're going to be dealing with really record-setting flooding in multiple regions."
Coastal water levels were already rising, as the first major storm of the annual Atlantic hurricane season forced hundreds of thousands to flee their homes and wreaked havoc on air travel.
As of 2300 GMT, Harvey was located about 45 miles east of Corpus Christi — a major hub for the American oil industry — and moving at eight miles an hour, the National Hurricane Center said, warning of possible "catastrophic" flooding.
The storm was expected to make landfall by early Saturday on the populous Texas coast, dumping up to 40 inches (more than 100 centimeters) of rain over the next four or five days and generating storm surges of up to 12 feet.
"All the advice we can give is get out, and get out now," said Patrick Rios, the mayor of Rockport, where a majority of the town's 9,500 residents had left.
Rios had blunt words for those determined to stay, telling them to "mark their arm with a Sharpie pen, put their social security number" — to be identified if found dead.
Highways leading from coastal areas were jammed as authorities issued mandatory evacuation orders in many areas.
Before the storm hit, the US Federal Emergency Management Agency (FEMA) prepositioned emergency supplies.
As he headed to the Camp David presidential retreat for the weekend with his family, Trump said: "Good luck to everybody."
Harvey is forecast to be the most powerful hurricane to hit the mainland since Wilma struck Florida in 2005, and could inflict billions of dollars in damage.
2005 was a huge year for hurricanes — before Wilma, Hurricane Katrina pummelled New Orleans, leaving more than 1,800 dead and becoming one of the greatest hiccups in the presidency of George W. Bush.
Coastal Texas is a fast-growing area, with some 1.5 million people moving into the area since 1999.
Authorities said the combination of dense growth and perhaps a year's worth of rain falling in just days could prove deadly.
Local television footage showed supermarket aisles plucked bare, houses and shops with windows boarded over, and long lines snaking outside gas stations.
The National Hurricane Center (NHC) warned of the "complete destruction of mobile homes," of many buildings "washing away," and some areas being left "uninhabitable for weeks or months."
In 2005, Bush faced severe criticism after FEMA appeared unprepared for the devastating damage inflicted by Hurricane Katrina.
"Keep on top of hurricane Harvey don't make same mistake Pres Bush made w Katrina," Republican Senator Chuck Grassley urged the US leader in a tweet.
In a series of tweets throughout Friday, Trump said he was closely monitoring the storm's progress and said he was "here to assist as needed."
"Storm turned Hurricane is getting much bigger and more powerful than projected. Federal Government is on site and ready to respond. Be safe!" he said after arriving at Camp David.
"This storm will likely be very destructive for several days," the White House said.
In Corpus Christi, a children's hospital evacuated 10 newborn infants to a facility inland due to the prospect of extended power outages.
But many Corpus Christi residents appeared bent on sitting the storm out, packing sandbags to protect their homes — until the supply of sandbags ran out.
Already, nearly 50,000 people in the city had lost power.
Sheriff Frank Osborne of Matagorda County, where evacuations were mandatory, described the high stakes.
"I'm not going to put one of my deputies' life on the line to save somebody that didn't leave when they were asked to," he told local TV station KHOU.
People east of the mammoth, three-mile (five-kilometer) seawall in Galveston Island were urged to evacuate. Galveston was the scene of a devastating category 4 hurricane in 1900 that claimed an estimated 8,000 lives.
Officials in Houston, the biggest city in the storm's path, canceled school but did not anticipate mass evacuations.
Inland cities like San Antonio prepared to welcome evacuees, and some began arriving on Friday.
FEMA chief Brock Long said the most pressing danger was the storm surge, the high tides powered by powerful winds, but said many inland counties should prepare for "significant" flooding.
One-third of the US refining capacity is potentially under threat. Crude prices rose slightly Friday ahead of landfall.
US authorities said about 22 percent of crude production in the Gulf of Mexico was shut down as of Friday mid-day — or more than 375,000 barrels a day. But total daily US production stands at 9.5 million barrels a day, experts say.
In Louisiana, Governor John Bel Edwards issued an emergency declaration for his entire state, as authorities in New Orleans — where Katrina did the most damage — readied high-water rescue vehicles and boats.
Edwards described a "worse-case scenario" in which the storm leaves Texas, gains new strength over the heated waters of the Gulf, and then heads toward Louisiana.
Meteorologist Eric Holthaus told AFP the prospect of the storm stalling on the coast, lashing it with heavy rain for days, was "just terrifying."
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