(Reuters) - Tropical Storm Don, the fourth named storm of the Atlantic hurricane season that formed on Wednesday over the oil-rich Gulf of Mexico, has not changed appreciably during the past several hours, heading west-northwest toward the coast of Texas, the U.S. National Hurricane Center said.
Tropical storm Don is currently located about 635 miles east-southeast of Corpus Christi, Texas, and about 590 miles east-southeast of Brownsville, Texas, the NHC said.
The NHC said a tropical storm warning for the coast of Texas has been extended southward to the mouth of the Rio Grande.
The center of Don should move through the southern and central Gulf of Mexico later Thursday and approach the Texas coast on Friday.
"Don is moving around the southern periphery of a ridge over the southern United States. This ridge is forecast to build slightly during the next day or two as a series of weak impulses move away from the northern Gulf Coast," the NHC added.
Don threatens the Gulf, home to thousands of oil and gas production platforms, producing about 29 percent of U.S. oil output and more than 10 percent of domestically produced natural gas.
Don, which currently has sustained winds near 40 miles per hour (64 km/h) with higher gusts, is expected to pack maximum sustained winds of 60 miles per hour (97 km/h) in the next 48 hours, said the NHC.
Several offshore energy operators, including Shell Oil Co, and Apache Corp have been forced to evacuate support workers on Wednesday, but have not undertaken measures to shut production yet.
Refiners along the Gulf Coast, home to 40 percent of the nation's refining capacity, were also watching the system.
The earlier aircraft measurements confirmed that Don is a rather small tropical cyclone. The next aircraft mission is scheduled for 12:00pm (local time).
The U.S. government's weather agency has forecast an "above normal" 2011 Atlantic hurricane season, spawning six to 10 hurricanes of which around half could become major.
(Reporting by Naveen Arul in Bangalore)