* Hurricane Ida damages Louisiana grain terminal, disrupts exports

* U.S. corn ratings at 60% good-to-excellent, soy steady at 56%

* Traders adjust positions on last day of month (Adds start of U.S. trading, latest prices, analyst comments, changes byline/dateline, pvs SINGAPORE/PARIS)

CHICAGO, Aug 31 (Reuters) - Chicago Board of Trade grain and soybean futures slumped to their lowest prices in weeks on Tuesday as damage to U.S. export facilities from Hurricane Ida stoked concerns among traders.

Uncertainty over the length of disruptions to shipments from the Gulf Coast, which accounts for about 60% of U.S. exports, rattled the markets at a time when global grain supplies are tight and demand from China is strong, analysts said.

In Louisiana, Ida damaged a Cargill Inc grain export elevator while rival shipper CHS Inc warned a facility may lack power for weeks.

"There's plenty of uncertainty about the Gulf," said Jim Gerlach, president of brokerage A/C Trading. "Nobody knows if it's something that's going to last for days, weeks or months."

Most-active soybean futures fell 17 cents to $12.86-1/4 a bushel by 11 a.m. CDT (1600 GMT) and hit their lowest price since Aug. 20. Corn slid 13-1/2 cents to $5.29-1/4 a bushel and set its lowest price since July 12. Wheat slipped 2-1/2 cents to $7.21 a bushel and touched its lowest price since Aug. 2.

Traders were adjusting positions on the last day of the month, brokers said.

On Monday, the U.S. Department of Agriculture rated 60% of the U.S. corn crop in good-to-excellent condition, unchanged from a week earlier and above analysts' expectations. For soybeans, the USDA rated 56% of the crop as good-to-excellent, steady from the previous week and matching trade expectations.

August rains have benefited the late development of crops, analysts said, after dryness earlier stressed fields in parts of the Midwest.

"The weather has improved this week and that is leading to some hope that the corn crop will finish on a positive note," said Tomm Pfitzenmaier, analyst for Summit Commodity Brokerage.

In Russia, the world's biggest wheat exporter, consultancy Sovecon cut its 2021 wheat crop estimate to 75.4 million tonnes from 76.2 million tonnes. (Reporting by Tom Polansek in Chicago, Naveen Thukral Singapore and Sybille de La Hamaide in Paris. Editing by Sherry Jacob-Phillips and Bernadette Baum)