Hurricane Isaac was barrelling across the Gulf of Mexico after skimming past south Florida. Based on its current track, it was due to hit the Gulf Coast between Florida and Louisiana Tuesday night or early Wednesday, the seventh anniversary of Hurricane Katrina, which devastated New Orleans.

The storm has already disrupted shipping: grain companies Cargill Inc and Archer Daniels Midland Co shut down some export elevators in Louisiana as a precaution.

Barge traffic along the Mississippi River between Baton Rouge, Louisiana, and the U.S. Gulf has also been suspended. The river is a major channel for the movement of grains produced in the Midwest farm belt to export terminals at the Gulf of Mexico for shipment around the world.

With corn stalk development hindered by the worst drought across the Midwest in 56 years, the high winds could flatten crops, analysts warned. Farmers have slowed harvests to allow crops to dry after recent rains. The latest USDA harvest progression report, released at the end of trading on Monday, showed the corn harvest was 6 percent complete as of Aug. 26, up just 2 percentage points from a week earlier, and below analyst expectations for 10 percent.

The five-year average for late August is 2 percent complete. A year ago, farmers had harvested 2 percent of the corn crop.

But Isaac could boost winter wheat plantings, with the World Weather Inc Forecasting the drought-parched Midwest farm belt could get up to 5 inches of rain.