Tropical storm Harvey has gained strength over the Gulf of Mexico and is still forecast to develop into a hurricane on Friday, the U.S. National Hurricane Center said in its latest update.

The storm, located about 600 km southeast of Port Mansfield, Texas, with maximum sustained winds of 70 km/r is likely to approach the Texas coast on Friday, the NHC said on Thursday.

Late Thursday morning, the NHC said the storm was likely to be a Category 3 hurricane when it makes landfall in Texas Friday morning with life-threatening storm surges and freshwater flooding expected. 

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

"Heavy rainfall is likely to spread across portions of eastern Texas, Louisiana, and the lower Mississippi Valley from Friday through early next week and could cause life-threatening flooding," the NHC said.

Emergency officials asked residents along the upper Texas coastline to move or prepare to move inland. Those in low-lying areas were urged to seek higher ground, and those elsewhere were told to monitor official announcements closely. 

On South Padre Island, people filled sandbags and loaded them into cars and vans 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.

Rainfall totals of 25 to 38 cm were possible over the middle and upper Texas coast and southwest Louisiana through Tuesday, the Miami-based hurricane centre said.

Royal Dutch Shell, Anadarko Petroleum and Exxon Mobil announced they were curbing some oil and gas output on at facilities in the Gulf of Mexico ahead of Harvey.

Tropical Weather Texas

Leo Sermiento, left, and Emilio Gutierrez, right, fill sandbags on South Padre Island, Texas. Texas Gov. Greg Abbott has ordered the State Operations Center to elevate its readiness level and is making state resources available for preparation and possible rescue and recovery actions. (Jason Hoekema/The Brownsville Herald/Associated Press)

Oil production slows

Shell said it was evacuating all personnel from the roughly 100,000 barrel-per-day Perdido oil and gas production platform as a precaution. Anadarko said it had shut in production and was evacuating workers from its Boomvang, Gunnison, Lucius and Nansen platforms in the Gulf of Mexico.

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.

The U.S. Gulf of Mexico is home to about 17 per cent of American crude oil output and 5 per cent 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.

With files from Reuters and CBC News