* Major hurricane forecast to hit South Texas coast Friday night
* Residents flee most powerful storm on U.S. mainland since 2005
* Locals told to take cover from wind, unprecedented flooding
* Nearly quarter of Gulf oil and gas output shut in (Adds weather in Corpus Christi in paragraph 5)
CORPUS CHRISTI, Texas, Aug 25 (Reuters) - Hurricane Harvey was hours away from lashing the Texas coast on Friday with winds of up to 156 miles per hour and 13-foot ocean surges as the most powerful storm in over a decade bore down on the mainland United States.
Harvey strengthened to a powerful Category 4 hurricane with cities from northern Mexico to Louisiana bracing for flooding, the National Hurricane Center said. The storm was expected to hit land near Corpus Christi, Texas, around 9 p.m. CDT (0200 GMT) then stall and dump over 3 feet (90 cm) of rain in areas of the Texas coast and parts of Louisiana as it lingers for days.
While thousands fled the expected devastating flooding and destruction, many residents defied mandatory evacuation orders and stocked up on food, fuel and sandbags.
"Were suggesting if people are going to stay here, mark their arm with a Sharpie pen with their name and Social Security number," Rockport Mayor Pro Tem Patrick Rios told reporters Friday, according to media reports. "We hate to talk about things like that. It's not something we like to do but its the reality. People dont listen."
As many as 5.8 million people were believed to be in the storm's path.
In Corpus Christi, a city of 320,000 under voluntary evacuation, strengthening winds buffeted the few trucks and cars that continued to circulate on the streets. The storm toppled wooden roadwork signs and littered the streets with pieces of palm trees as white caps rocked sailboats in their docks.
About 85 miles (137 km) north in Victoria, Mayor Paul Polasek told CNN he estimated that 60 percent to 65 percent of the towns 65,000 residents defied the mandatory evacuation order.
Jose Rengel, a 47-year-old who works in construction, said he was one of the few people in Jamaica Beach in Galveston that did not heed a voluntary evacuation order.
All the shops are empty, he said as the sky turned black and rain fell. Its like a tornado went in and swept everything up.
About 20,000 passengers were riding out the storm aboard three cruise ships in the Gulf of Mexico.
As a Category 4 hurricane on the Saffir-Simpson scale, Harvey could uproot trees, destroy homes and disrupt utilities for days. If it maintains its intensity, it would be the first major hurricane to hit the mainland United States since Hurricane Wilma struck Florida in 2005.
The storm was about 45 miles (70 km) off Corpus Christi and packing winds of 130 mph (215 kph), 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.
"Life-threatening and devastating flooding expected near the coast due to heavy rainfall and storm surge," the NHC said.
President Donald Trump, facing the first large-scale natural disaster of his presidency, on Friday tweeted: "I am closely watching the path and doings of Hurricane Harvey ... BE SAFE!" Earlier, a White House official said the president was considering a request to issue an emergency declaration, providing federal disaster relief.
Louisiana and Texas declared states of disaster, authorizing the use of state resources to prepare.
The city of Houston warned residents of flooding from close to 20 inches (60 cm) of rain over several days.
Houston Mayor Sylvester Turner advised city residents not to leave en masse, saying "no evacuation orders have been issued for the city." Chaotic traffic from a rushed evacuation in 2005 with Hurricane Rita proved tragic. "Calm and care!" he said in a tweet.
GASOLINE PRICES SPIKE
Gasoline stations on the south Texas coast were running out of fuel as thousands of residents fled the region. U.S. gasoline prices spiked as the storm shut down 22 percent of Gulf of Mexico oil production, according to the U.S. government.
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."
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 and Royal Dutch Shell Plc, Anadarko Petroleum Corp, Exxon Mobil Corp and others have evacuated staff from offshore oil and gas platforms.
Concern that Harvey could cause shortages in fuel supply drove benchmark gasoline prices to their highest in four months, before profit taking pulled back prices. Meanwhile, U.S. gasoline margins <RBc1-CLc1> hit their strongest levels in five years for this time of year.
The U.S. government said it would make emergency stockpiles of crude available if needed to plug disruptions. It has regularly used them to dampen the impact of previous storms on energy supplies.
(Reporting by Brian Thevenot; Additional reporting by Ernest Scheyder and Marriana Parraga in Houston, Brian Thevenot in Corpus Christi, Texas; and Devika Krishna Kumar in New York; Writing by Lisa Shumaker; Editing by James Dalgleish and Mary Milliken)