GRAINS-Wheat, soy futures fall on firmer dollar, U.S. rains

* Dollar hits 13-month high

* Concern over U.S. export prospects as big crops loom

(Updates prices, adds analyst comments; changes bylines, dateline, previous HAMBURG) CHICAGO, Aug 15 (Reuters) - U.S. grains and oilseed futures fell on Wednesday, pressured by broad selling in commodities and equities as a firmer dollar prompted worries about export prospects for American supplies, traders and analysts said. Wheat prices notched some of the biggest declines among agriculture products, dropping after rising slightly during the previous session. Rainfall in parts of the southern U.S. Plains boosted soil moisture ahead of the winter wheat planting season, further weighing on the market. "Today is a macro day. Crude oil is down, stock markets are down," said Price Futures Group analyst Jack Scoville. "We're getting a bunch of rain in Kansas; we've gotten some in Oklahoma ... some of those wheat areas are getting a lot of rain and that'll be good for the next crop," he added.

Chicago Board of Trade September wheat was down 8-3/4

cents at $5.33 per bushel at 11:25 a.m. CDT (1625 GMT), above its earlier 2-1/2 week low of $5.28-1/4. Prices for wheat, corn and soybeans have lost ground since a bearish U.S. Department of Agriculture monthly crop report on Friday predicted bigger-than-expected U.S. soy and corn harvests as well as larger global wheat supplies. "(USDA) on Friday made bold forecasts of large U.S. soybean and corn yields so the dollar strength is unwelcome against the expectation that big U.S. harvests will need to find buyers on world markets," said Charles Clack, agricultural commodity analyst at Rabobank.

The dollar hit a 13-month peak against a basket of

currencies. The greenback strengthened against the Russian ruble after Washington announced fresh sanctions against Moscow, making Russian grain comparatively cheaper in the world market. Top global wheat buyer Egypt on Tuesday bought 420,000 tonnes of wheat from Russia and Romania in a tender.

The continuing trade dispute between the United States and China is also burdening markets. "There is also worry about lack of real progress to solve the U.S.-China trade war with the question mark remaining over the future of U.S. soybean exports to China," Clack said. USDA will release weekly U.S. export sales data on Thursday.

CBOT November soybeans were down 6-3/4 cents to $8.73 per bushel and CBOT December corn down 1 cent to

$3.75-1/2. U.S. processors crushed a bigger-than-expected 167.733 million bushels of soybeans during July, their second largest monthly total ever, the National Oilseed Processors Association said on Wednesday.

(Additional reporting by Michael Hogan in Hamburg and Colin Packham in Sydney Editing by Tom Brown)