Tropical depression Harvey is headed for Texas and could develop into a hurricane just before landfall.
Harvey, which could become the first hurricane to strike Texas since 2008, has forced workers to be evacuated from Gulf of Mexico platforms.
Harvey was 755km southeast of Port Mansfield, Texas, with top winds of 55 km/h, the US National Hurricane Centre said in an advisory.
"It could intensify right up to landfall on [Saturday NZT]," said Jeff Masters, co-founder of Weather Underground in Ann Arbor, Michigan. "I expect a Category 1 hurricane at landfall, but I cannot rule out a Category 2."
The Gulf Coast from Corpus Christi, Texas, to Lake Charles, Louisiana, is home to nearly 30 refineries - making up about 7 million barrels a day of refining capacity - and is in the path of heavy rainfall. Flooding poses risks to operations, while torrential rains can shut units and cause supply disruptions.
Fuel supplies in the region may be tightened further by other refinery outages. Phillips 66 began a plantwide shutdown of its Lake Charles, Louisiana, refinery after its power supplier warned of a high potential for electrical failure, according to a company statement.
Ike in 2008 was the last hurricane to hit Texas, said Dennis Feltgen, spokesman for the National Hurricane Centre. Ike struck as a Category 2 storm on the five-step Saffir Simpson scale.
Along the coastline, seas could rise 1.2m to 1.8m above ground level and from 10 to 15 inches of rain will probably fall across parts of Texas into Louisiana, the hurricane centre said. Some areas could get as much as 20 inches of rain.
"It is going to be a wet one," Masters said. "It is not going to move fast after landfall and that is going to cause big trouble" from flooding rains.
The current track calls for the storm to land in southeastern Texas and then turn toward Houston. Masters said at least one computer-forecast model shows the storm heading back into the Gulf of Mexico early next week before coming ashore in Texas again.
A hurricane watch was issued for the coast from Port Mansfield at the south to San Luis Pass, at the western end of Galveston Island, at the north. A storm surge watch was in effect for Port Mansfield to High Island, just up the coast from Galveston.
A tropical storm watch was in effect for Boca de Catan, Mexico, just south of the Texas border, to Port Mansfield and from San Luis Pass to High Island.
Texas Governor Greg Abbott ordered the State Operations Centre to elevate its readiness level, making state resources available for possible rescue and recovery actions amid forecasts the tropical storm would make landfall along the state's Gulf Coast.
- additional reporting AP