Hurricane Harvey, taking dead aim on the Texas Gulf coast, strengthened to a Category 4 storm Friday only hours before it was expected to roar ashore and unleash several days of high winds, flash flooding, storm surges and up to three feet of rain.

"This is going to be a very major disaster," said Gov. Gregg Abbott.

Fueled by the warm Gulf waters, Harvey jumped within hours from a Category 2 to Category 3 hurricane Friday afternoon. It jumped again at about 4 PT to a Category 4. It was expected to make landfall overnight with winds of at least 130 mph.

The storm is forecast to make landfall in Texas late Friday or early Saturday.

With some 700,000 people living in the hurricane warning zone — roughly half of them around Corpus Christi — traffic backups were reported on heavily traveled roads such as Interstate 37 as people move toward San Antonio and other inland locations.

"Texas is about to have a very significant disaster," Brook Long, director of the Federal Emergency Management Agency, told CNN.

If people have not already left the most threatened areas, he said, "their window to evacuate is rapidly coming to a close."

The hurricane looks to hit about the same area as one of the strongest and deadliest hurricanes to ever hit the U.S.: the Indianola Hurricane of 1886.

The Associated Press reports the National Hurricane Center says the Indianola hurricane ranks as the fifth strongest hurricane to make U.S. landfall, behind the 1935 Keys hurricane, 1969's Camille, 2006's Katrina and 1992's Andrew. About 150 people died, putting it in the top 25 most fatal hurricanes.

MIT meteorology professor Kerry Emanuel says Indianola was a thriving city before it was struck and it never came back. According to Texas Escapes magazine, it's now pretty much a ghost town.

It would be the strongest hurricane to hit the U.S. in 12 years. A major hurricane is one that's a Category 3 or above on the Saffir-Simpson scale of hurricane intensity.

All seven Texas counties on the coast from Corpus Christi to the western end of Galveston Island have ordered the mandatory evacuations of tens of thousands of residents from low-lying areas.

Voluntary evacuations have been urged for Corpus Christi and for the Bolivar Peninsula, where many homes were washed away by the storm surge of Hurricane Ike in 2008.

Texans along the coast awoke Friday to the first pelting rains of Harvey, as the storm continued its menacing march toward them.

Corpus Christi residents said they worried most about the dangerous amounts of rain and flooding Harvey is expected to dump on the region, as it’s forecast to become wedged between two pressure systems when it makes landfall late Friday.

Meanwhile, they sandbagged homes, pulled boats out of the water, brought in last-minute supplies or headed out of the danger zone.

Businesses and homes along Port Aransas, one of the favorite beach spots along the coast, sat boarded up and quiet late Thursday as residents heeded its mandatory evacuation. The ferry to the barrier island was expected to cease operations on Friday morning.

Forecasters were measuring 28-foot-high waves in the eye of the storm as residents in the Port Lavaca and Rockport areas braced for some of the worst of Harvey’s floods. The storm’s eye was expected to land just to the south of them, sending walls of water their way.

“It doesn’t matter the intensity, it’s the rain we’ll be looking at,” Meteorologist Joe Gazan of KIII-TV warned residents early Friday.

Corpus Christi Mayor Joe McComb and county officials stopped short of ordering a mandatory evacuation for the city, but they said residents who stay are risking their lives.

“There will come a point during the storm where rescue operations will cease,” McComb said at a Thursday news conference. “Please don’t put our public safety officers at risk.”

As Harvey strengthened in the Gulf of Mexico, the hurricane center issued a hurricane warning for a 280-mile stretch of the Texas coast on Thursday and forecast up to 25 inches of rain over the next week. Some 700,000 people live where a hurricane warning is in effect, roughly half of them in the Corpus Christi area.

Forecasters expected the storm system to be either slow-moving or possibility stationary for three to five days, which heightened concerns over heavy rainfall.

If this materializes, the National Weather Service in Houston said, some areas could see dangerous and life-threatening flash flooding.

"Since Harvey is forecast to stall, we expect 10-20 inches of rain over a large part of southern and eastern Texas and southwestern Louisiana from Friday into early next week," according to AccuWeather hurricane expert Dan Kottlowski.

The hurricane warning covers an area from Port Mansfield in the central coastal area to Matagorda at the tip near the Mexican border.

"Impacts from Harvey will be tremendous in terms of displacement of people, property and economic loss and travel and freight disruptions," according to AccuWeather's Marshall Moss.

White House press secretary Sarah Huckabee Sanders said the hurricane was “certainly something he [President Trump] is aware of.”

She said the lack of a permanent Homeland Security secretary to replace new Chief of Staff John Kelly would not inhibit the response. “There's certainly someone at the helm. You have acting secretary Elaine Duke, who's watching this closely and is involved in the process along with the FEMA director,” she said.

In Kelly, she said, there is “no better chief of staff for the president during the hurricane season.”

As the storm moved toward the mainland, Royal Dutch Shell, Anadarko Petroleum and Exxon Mobil were already evacuating workers and reducing production of soil and gas at some facilities.

The U.S. Gulf of Mexico is home to about 17% of American crude oil output and 5% of dry natural gas output, according to the U.S. Energy Information Administration. More than 45% of the nation's oil refining capacity is along the U.S. Gulf Coast.

The last major hurricane to hit Texas was Ike in September 2008, which brought winds of 110 mph in the Galveston and Houston areas. Ike left damages of $22 billion.

Texas Gov. Greg Abbott has ordered the State Operations Center to raise its readiness level, making state resources available for possible rescue and recovery actions. The governor also pre-emptively declared a state of disaster for 30 counties on or near the coast to speed deployment of state resources to any affected areas.

Emergency officials have urged residents along the upper Texas coastline to move or prepare to move inland. Those in low-lying areas should seek higher ground, officials said.

Contributing: Julie Garcia and Matt Woolbright Corpus Christi Caller-Times; Gregory Korte, USA TODAY; Associated Press