Aug. 3 (Bloomberg) -- A tropical disturbance, a storm system that can precede a tropical storm or hurricane, formed today in the northern Gulf of Mexico about 150 miles (241 kilometers) south of Pensacola, Florida, U.S. forecasters said.

``Environmental conditions are marginally favorable for additional development, and a tropical depression could form in the next day or so as the system moves toward the west,'' according to a statement issued by the U.S. National Hurricane Center at 11 a.m. Miami time.

An Air Force aircraft known as a hurricane hunter will investigate the storm this afternoon, the Miami-based center said. It recommended those with interests in the northwestern Gulf to monitor the storm.

Exxon Mobil Corp.,Chevron Corp. and Royal Dutch Shell Plc are among oil producers and drillers that evacuated personnel from offshore rigs and platforms when Dolly, the season's first hurricane in the Gulf, swept through the region in late July.

The U.S. Gulf of Mexico produces about 1.3 million barrels a day of crude oil and 7.7 billion cubic feet of natural gas, according to the U.S. Minerals Management Service, part of the Interior Department.

A disturbance is a tropical storm that has developed a structure resembling more powerful storms and is maintained for a day or more.

A depression is a tropical cyclone with winds of 38 mph or less. Tropical storms are named once sustained winds reach 39 mph. The threshold for a hurricane is 74 mph. The next Atlantic tropical storm will be called Edouard.

To contact the reporter on this story: Margot Habiby in Dallas at mhabiby@bloomberg.net.

Last Updated: August 3, 2008 12:06 EDT