Fallout from Tropical Storm Isaac is likely to include drought-relieving rainfall for a big chunk of the central and southern U.S. Midwest, an agricultural meteorologist said on Monday.

Isaac was about 400 miles southeast of the mouth of the U.S. Mississippi River early on Monday and, according to Dee, should make landfall early Wednesday on either the Mississippi or Louisiana Gulf Coast as a Category 1 hurricane with 90-mile-per-hour winds.

A side benefit to the storm will be welcome rainfall to parched U.S. crop land and grazing lands.

The worst drought in more than a half-century has already harmed much of the nation's corn and soybean crops. While the wet weather as a fallout from Isaac will be welcome, it is too little too late to be a huge benefit to this year's crop.

"The big question is how much will later planted soybeans benefit," said John Dee, meteorologist for Global Weather Monitoring. "It would have helped a lot more if the rainfall came two weeks ago."

Dee said from 3 to 5 inches or more rainfall was expected beginning on Wednesday and Thursday in Louisiana and Mississippi, from 1 to 4 inches by Friday in Arkansas and Missouri, and from 0.50 to 1.00 inch or more by Saturday and Sunday in Missouri, Illinois, Indiana and Ohio.

"It will certainly add valuable soil moisture for fall planting of the winter wheat crop," Dee said. "Also, Kansas and Oklahoma received good rains of 1.00 to 2.00 inches over the weekend, which will help plantings of hard red winter wheat there."

Dee said the remnants from Isaac, while providing much-needed moisture, were not likely to harm mature crops in the southern or central U.S. Midwest.

"There will be some wind, but I don't think there will be much harm to crops," he said. "It will slow harvest for a few days."

The annual Pro Farmer tour of Midwest crops found signs of severe crop losses in the top-producing Midwest crops. On Friday it forecast the U.S. corn and soybean harvest smaller than the government is predicting, with production the lowest in nearly a decade.

Additionally, a Reuters poll of 11 analysts estimated the 2012 U.S. corn yield per acre at 121.5 bushels, the lowest in 16 years, and production at 10.5 billion bushels, an eight-year low.

In its first survey asking for estimates of the amount of corn to be harvested compared with plantings, the poll showed the percentage of harvested corn area at the lowest in nine years.

Analysts' expectations for corn production this season fell 3 percent below the U.S. government's forecast earlier in August and 6 percent below a similar poll of analysts taken by Reuters at the end of July.

Late on Monday, the U.S. Department of Agriculture will release updated harvest progress reports in its weekly crop progress report. Last Monday, the USDA said 4 percent of the U.S. corn crop had already been harvested, the fastest start ever. The crop was planted early and then was pushed to maturity by relentless heat and drought over the summer.

(Reporting by Sam Nelson; Editing by Lisa Von Ahn)