CHICAGO--Major grain-trading companies shut down grain elevators in Louisiana to take precautions as Tropical Storm Isaac advanced toward the U.S. Gulf Coast.

Archer Daniels Midland Co. (ADM) closed four elevators and its port operations in New Orleans, and rival Cargill Inc. said it was working on closing two elevators near the city.

The U.S. Coast Guard also closed the Port of New Orleans to all vessels at 3:15 p.m. EDT Monday, a spokesman said. Isaac is on pace to become a hurricane and unleash potentially serious flooding by the time it makes landfall Tuesday or Wednesday.

"ADM is monitoring Tropical Storm Isaac very closely," a spokeswoman for Decatur, Ill.-based ADM said Monday. "We have activated our hurricane-readiness plan and are taking precautions to ensure the safety of our employees and their families."

Cargill said it will effectively close elevators in Reserve and Westwego, La.

"We'll be securing equipment and facilities, but in effect be [closed]...so our employees can get home," a Cargill spokesman said Monday.

Cargill, one of the biggest closely held U.S. companies by revenue, is based in suburban Minneapolis.

The Louisiana Gulf is a major export hub for U.S. corn, soybeans and wheat. Grains from Midwest farms are sent on barges down the Mississippi River, where they are shipped around the world. Most of the U.S. corn and soybean harvest will begin in the coming weeks.

The National Hurricane Center estimated Tropical Storm Isaac was around 280 miles from the mouth of the Mississippi River as of 2 p.m. EDT, moving northwest at about 14 miles per hour and carrying winds of up to 65 mph.

Though Isaac was expected to become only a low-level Category One hurricane with winds around 90 miles per hour by the time it comes onshore, forecasters warned it could push a substantial amount of water into low-lying areas, with a storm surge of six to 12 feet in coastal Alabama, Mississippi and Louisiana.

--Timothy W. Martin and Mike Esterl contributed to this article.

Write to Andrew Johnson Jr. at andrew.johnsonjr@dowjones.com.

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