Harvey attained hurricane status early Thursday afternoon, with winds of 80 mph, the National Hurricane Center said.
Harvey is now forecast to hit the Texas Gulf Coast as a Category 3 hurricane with winds of 115 mph. This would be the strongest hurricane to hit the U.S. in 12 years — since Hurricane Wilma in October 2005.
As 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.
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.
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 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.
As of 1 p.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 80 mph.
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.
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