(Updates prices, adds comments; changes byline, dateline, previously SYDNEY/PARIS)
* Rains forecast for Midwest corn and soybean crops
* Wheat down on hefty global supplies
* Soyoil firm ahead of expected decision on U.S. imports
CHICAGO, Aug 21 (Reuters) - U.S. grain prices fell more than 1 percent on Monday, with benchmark Chicago Board of Trade December corn sinking to a nearly one-year low, on technical selling and easing concerns that dry weather would reduce harvests.
Several CBOT wheat futures contracts dropped to lifetime lows, while soybean prices were narrowly mixed.
Soyoil and palm oil futures both gained ahead of the U.S. Department of Commerce's expected announcement at midday Tuesday of countervailing duties on imports of Argentine and Indonesian biodiesel.
"There's not many areas left in the United States that will have substantial drought impact, and there's a lack of bullish news (in wheat)," said Futures International analyst Terry Reilly.
Rainfall was expected to benefit soybeans and corn in the top growing state of Iowa, INTL FCStone said in a client note.
Scouts on an annual U.S. crop tour found variable yields for corn and soybean crops in parts of Ohio and South Dakota.
Analysts surveyed by Reuters expected the U.S. Department of Agriculture to show improved U.S. soybean crop condition ratings and unchanged evaluations of corn and spring wheat.
"Climatic conditions have improved, and whatever threat there is to some crops has already been priced in," said National Australia Bank agribusiness economist Phin Ziebell.
CBOT December corn was down 2-3/4 cents at $3.63 per bushel at 12:15 p.m. CDT (1715 GMT), near the contract's lifetime low of $3.58-1/2 reached on Aug. 31, 2016. Corn on a continuous chart was the lowest since June 27.
CBOT December wheat dropped to a contract low of $4.35-1/2 per bushel, before trimming losses to $4.38, down 4-1/2 cents.
CBOT November soybeans were down 1 cent at $9.36-3/4 per bushel. Losses were limited in part by the USDA's announcement that exporters sold a total of 661,000 tonnes of U.S. soybeans to China and unknown destinations for the 2017-18 season, which starts on Sept. 1.
September soyoil futures were up 0.34 cent, or 1 percent, at 33.95 cents per pound. Investors were squaring their positions ahead of the government announcement of import duties, Reilly said. (Additional reporting by Colin Packham in Sydney and Sybille de La Hamaide in Paris; Editing by Lisa Von Ahn)