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UPDATE 8-Hurricane Harvey strengthens, threatens record flooding in Texas

* Hurricane forecast to hit Gulf coast late Friday or Saturday

* Residents fleeing most powerful storm on U.S. mainland since 2005

* Locals told to take cover from wind, floods

* Oil and gas operations already affected

* Schools closed, flights canceled (Updates with earlier landfall, record-rain amounts, president's advisory)

CORPUS CHRISTI, Texas, Aug 25 (Reuters) - Hurricane Harvey intensified on Friday and was poised to hit the mainland United States this evening as the most powerful storm in more than a decade and authorities warned residents to shelter from life-threatening winds and flooding.

Harvey became a Category 3 hurricane, the third most powerful, the National Hurricane Center said on Friday. Forecasters said the storm would hit land near Corpus Christi, Texas, around 8 p.m. or 9 p.m. CDT (0100-0200 GMT), dropping over three feet of rain in some areas along the central Texas coast, and potentially loop back over the Gulf of Mexico before hitting Houston.

"My urgent message to my fellow Texans is that if you live in a region where evacuation has been ordered, you need to heed that advice and get out of harm's way while you can," Texas Gov. Greg Abbott said in a televised address.

U.S. gasoline prices spiked as the storm shutdown 22 percent, or 377,000 barrels per day (bpd) of Gulf of Mexico oil production and halted 4.4 percent of U.S. refinery output, according to the U.S. government.

"We may be looking at unprecedented and major to record flooding," said forecaster John Tharp of Weather Decision Technologies,

Sea levels may surge as high as 12 feet (3.7 meters) and flood warnings are in effect for Louisiana and northern Mexico, Tharp said.

Gas stations and grocery stores in the region were packed as residents readied their cars and pantries for any shortages following the storm. Coldplay, the British rock band, canceled a Friday concert in Houston, telling fans it didn't want to risk anyone's safety.

At a Willis, Texas, station, about 50 miles (77 km) north of Houston, Corey Martinez, 40, was heading to Dallas from his Corpus Christi home.

"It has been pretty stressful. We're just trying to get ahead of the storm," he said. "We've never been through a hurricane before."

As a Category 3 hurricane on the Saffir-Simpson scale, Harvey could bring winds of 111-129 miles per hour (178-208 km/h) that can uproot trees, rip roofs off homes and disrupt utilities for days. It it maintains its intensity, it would be be the first major hurricane to hit the mainland United States since Hurricane Wilma struck Florida in 2005.

The storm was about 85 miles (140 km) off Corpus Christi and packing winds of 110 mph on Friday afternoon, the NHC said. The NHC's latest tracking model shows the storm sitting southwest of Houston for more than a day, giving the nation's fourth most populous city a double dose of rain and wind.

"Now is the time to urgently hide from the wind. Failure to adequately shelter may result in serious injury, loss of life, or immense human suffering," the National Weather Service said.

"Life-threatening and devastating flooding expected near the coast due to heavy rainfall and storm surge," the NHC said.

President Trump on Friday tweeted: "I encourage everyone in the path of #HurricaneHarvey to heed the advice & orders of their local and state officials." A White House official said the president was considering a request to issue an emergency declaration, providing federal disaster relief.

The storm's approach triggered evacuations in south Texas communities and central coast residents were voluntarily leaving the area. Cities canceled classes on Friday and Monday at dozens of schools along the south Texas coast, home to 5.8 million people from Corpus Christi to Galveston.

David Ramirez left his home in Corpus Christi early on Friday to wait out the storm in San Antonio, Texas.

"With the level of storm surge they're talking about, there isn't a lot I could do to protect my house," he said in an interview while awaiting directions to an emergency shelter.

Harvey also forced the cancellation or delay of at least 40 flights in and out of major airports in Texas on Friday, according to Flightaware.com, which tracks airline traffic.

Louisiana and Texas declared states of disaster, authorizing the use of state resources to prepare. President Donald Trump has been briefed and is ready to provide resources if needed, the White House said on Thursday.

The port of Houston, the nation's busiest petrochemical port, closed its terminals at noon, and earlier halted inbound and outbound ship traffic on Friday. The city of Houston warned residents of flooding from close to 20 inches of rain over several days.

GASOLINE PRICES SPIKE

More than 45 percent of the country's refining capacity is along the U.S. Gulf Coast, and nearly a fifth of the nation's crude oil is produced offshore. Ports from Corpus Christi to Texas City, Texas, were closed to incoming vessels.

The U.S. government said 9.6 percent of crude output capacity was shut and 14.6 percent of natural gas production was halted.

Three refineries in Corpus Christi and one farther inland at Three Rivers were shutting down ahead of the storm. Two others reduced output as ports were closed.

Concern that Harvey could cause shortages in fuel supply drove benchmark gasoline prices to their highest in four months, before profit trading pulled back prices. Meanwhile, U.S. gasoline margins <RBc1-CLc1> hit their strongest levels in 5 years for this time of year earlier in the day.

The U.S. government has emergency stockpiles of crude available to plug disruptions, and has regularly used them to dampen the impact on energy supplies of previous storms.

Houston-based energy bank Tudor Pickering Holt & Co said in a note not to expect significant or lasting production impacts from Harvey. But it said it would impact some production and disrupt refinery runs, imports and exports, "which will show up in the weekly inventory numbers for the next few weeks."

Royal Dutch Shell Plc, Anadarko Petroleum Corp and Exxon Mobil Corp have evacuated staff from offshore oil and gas platforms in the storm's path.

The potential for flooding at shale oil fields in south Texas that produce more than one million barrels of oil a day led several producers to curb operations. EOG Resources Inc said shut some production in the Eagle Ford shale region. Noble Energy Inc and Statoil ASA also said they were evacuating some staff from production facilities.

(Reporting by Brendan O'Brien in Milwaukee and Brian Thevenot in Corpus Christi; Editing Meredith Mazzilli and Andrew Hay)