GRAINS-U.S. wheat falls; corn, soy firm but all post weekly loss

(Updates with closing prices, adds new analyst quote; changes byline, dateline; previous PARIS/SINGAPORE)

CHICAGO, Aug 11 (Reuters) - U.S. wheat futures fell on Friday, led lower by a sharp decline in MGEX spring wheat as investors liquidated bullish positions following the U.S. Department of Agriculture's forecast for a bigger-than-expected crop in the northern Plains, traders said.

The selloff pushed the benchmark Chicago Board of Trade soft red winter wheat contract to its lowest in nearly two months. MGEX spring wheat ended down 4.2 percent and hit its lowest since June 26, pressured by the USDA's outlook for a bigger-than-expected crop in the northern Plains.

Chicago soybean and corn futures edged higher, with mild bargain buying underpinning prices.

All three posted weekly declines as the USDA's forecast on Thursday for bumper corn and soybean harvests continued to hang over the market.

Wheat futures have fallen for five straight weeks, and corn and soybeans for three.

Chicago Board of Trade September soft red winter wheat futures ended down 1-1/4 cents at $4.39-1/4 a bushel. MGEX spring wheat for September delivery shed 29-1/4 cents to $6.74 a bushel.

"From a technical standpoint, I don't like the trade action, breaking that $7 number," said Jason Ward, analyst with Northstar Commodity Investment Co. "That was a very big support area that broke."

CBOT November soybeans were 4-3/4 cents higher at $9.45 a bushel and CBOT December corn was up 3-3/4 cents at $3.74-3/4 a bushel.

The USDA on Thursday projected the U.S. corn yield at 169.5 bushels per acre (bpa), below its previous forecast of 170.7, but well above the average trade estimate of 166.2 bpa.

The USDA also wrong-ooted investors by increasing its soybeans yield forecast to 49.4 bpa from 48.0, rather than trimming it as expected.

Traders have been grappling with erratic crop weather that has left some crop belts parched and others soggy. Rainfall was forecast to be limited this week in much of the Midwest, but heavy rain was projected toward the end of next week.

"Market attention now turns back to weather, with yield potential for U.S. soybeans and corn remaining the key focal point for the next month," Rabobank analysts said in a research note.

Wheat markets remained dampened by the USDA's supply outlook, which included a sharp increase in projected global wheat stocks in 2017-18, supported by increased estimates for Russian, Ukrainian and Kazakh crops.

Russian consultancies SovEcon and IKAR on Friday gave similar estimates for the world's biggest wheat exporter last season. (Additional reporting by Julie Ingwersen in Chicago, Naveen Thukral in Singapore and Gus Trompiz in Paris; editing by David Evans and Andrew Hay)