HOUSTON - U.S. refiners said operations were normal at refineries in Texas and Louisiana on Friday morning as they monitored a storm system in the western Gulf of Mexico that has a low chance of becoming a tropical cyclone.

Valero Energy Corp spokesman Bill Day said contingencies were in place in case thunderstorms in the western Gulf should become organized into a tropical depression or storm over the weekend.

Valero has refineries stretching from Corpus Christi, Texas, to Norco Louisiana, just west of New Orleans.

Exxon Mobil Corp spokesman Kevin Allexon said the company was monitoring weather across the Gulf of Mexico.

At this time, our facilities are operating normally, Allexon said.

Exxon's refineries in Baytown, Texas, and Baton Rouge, Louisiana, are the first and second largest refineries in the United States. The company also has a refinery in Beaumont, Texas.

Lyondell Basell ACCELC.UL spokesman David Harpole said operations at the company's Houston refinery had not been altered due to the weather.

The U.S. National Hurricane Center said said a mid- to upper-level trough in the western Gulf was causing rain and thunderstorms along the Gulf Coast from the Texas-Mexico border to western Louisiana.

There is a low chance, less than 30 percent, of this system becoming a tropical cyclone during the next 48 hours, the National Hurricane Center said in its Friday morning tropical weather outlook.

So far in 2009, no tropical storms have threatened the U.S.-regulated oil production areas in Gulf of Mexico or at refineries in Texas, Louisiana and Mississippi.

The Gulf accounts of a quarter of national oil production and 15 percent of U.S. natural gas output.

Refineries stretching from Corpus Christi to Pascagoula, Mississippi, account for 38.4 percent of U.S. refining capacity. (Reporting by Erwin Seba; Editing by John Picinich)