As Tropical Storm Harvey strengthened in the Gulf of Mexico, the National 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.

The storm is expected to make landfall as a major hurricane (Category 3 and above) with winds of 115 mph. This would be the first major hurricane to hit the U.S. since Wilma in 2005.

The threat of heavy rainfall is increased by forecasts that the storm system will be either slow moving or possibility stationary for 3 to 5 days.

If this materializes, the National Weather Service in Houston said, some areas could see extremely high rainfall totals leading to 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.

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 per cent of the nation's oil refining capacity is along the U.S. Gulf Coast.

The forecast track shows Harvey strengthening to a hurricane — meaning sustained winds of 74 mph or higher — late Thursday as it treks across the Gulf, making landfall along the southern Texas coast Friday evening or Friday night.

The threat of heavy rainfall is intensified by the prospect, still remote, that Harvey could stall for two or three days or even re-emerge early next week over the Gulf, restrengthen and make a second landfall in Louisiana, according to The Weather Channel.

As of 11 a.m. EDT, the storm was centered about 365 miles southeast of Corpus Christi and was moving north-northwest at 10 mph.

The storm’s maximum sustained winds had increased to near 65 mph. The U.S. National Hurricane Center said additional strengthening was expected and Harvey could become a hurricane by Friday, when it’s expected to approach the southern Texas coast.

A hurricane watch was in effect for Texas from Port Mansfield to San Luis Pass. A storm surge watch was in effect from Port Mansfield to High Island. Forecasters said the storm surge could be from 5 to 7 feet from Port Mansfield to San Luis Pass.

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.

Exxon was in the process of reducing production at its Hoover facility in the Gulf of Mexico, company spokeswoman Suann Guthrie said. The company said it was also working on transportation plans for staged evacuation of its personnel from its offshore facilities, expected to be in the path of the storm, to shore.

On South Padre Island, people filled sandbags and loaded them into cars and vans Wednesday to take to protect exposed homes and businesses. Others in the forecast path of the storm sought out generators, plywood and other goods from hardware stores. Meanwhile, rice farmers in coastal Matagorda County moved quickly to harvest their crops.

Contributing: Julie Garcia, Corpus Christi Caller-Times; Associated Press