GRAINS-Soybeans, wheat firm on strong exports; corn weakens on weather

* Forecast calls for more rain in U.S. Midwest

* Corn, soy, wheat all post monthly losses

(Updates with closing prices) CHICAGO, July 31 (Reuters) - U.S. soybean futures rose on Thursday on signs that demand will keep domestic supplies tight until harvest, while corn weakened due to forecasts for more crop-boosting rain in the U.S. Midwest, traders said. "The beans firmed up due to some speculators taking advantage of the weakness earlier in light of the very strong demand we are seeing," said Terry Reilly, an analyst at Futures International in Chicago. "It is basically a domestic- and export-demand scenario that is driving this market higher." Wheat also rose, which traders attributed to strengthening export prospects due to concerns about quality in the European crop as well as some end-of-month short-covering. Chicago Board of Trade soybeans for August delivery settled up 4 cents at $12.24-1/2 a bushel. CBOT November soybeans, which track what is expected to be a record U.S. crop, were up just 3/4 cent at $10.82 a bushel. The U.S. Agriculture Department said on Thursday morning that old-crop export sales of soybeans were 187,400 tonnes in the latest reporting week, near the high end of the range of forecasts for 100,000 to 200,000. New-crop soybean export sales were 1.27 million tonnes, bigger than expected. USDA also reported weekly wheat export sales of 801,000 tonnes, topping market forecasts. CBOT September corn was 4-3/4 cents lower at $3.57 a bushel, while CBOT September wheat was up 3 cent at $5.30-1/4 a bushel. "The weather and the forecast for rains next week is continuing to cap the corn market," said Craig VanDyke, analyst at Top Third Ag Marketing. "There's not a lot of weather premium in the corn market but we need the rain to put in the top end of production." Soybeans fell 12.6 percent in July, on track for their biggest monthly decline since September 2011. Corn was down 15.8 percent for the month, while wheat dropped 6.1 percent. Prices at 2:13 p.m. CDT (1913 GMT)

LAST NET PCT YTD CHG CHG CHG CBOT corn 357.00 -4.75 -1.3% -15.4% CBOT soy 1223.75 4.00 0.3% -6.8% CBOT meal 391.30 3.70 1.0% -10.6% CBOT soyoil 36.17 0.06 0.2% -6.8% CBOT wheat 530.00 3.00 0.6% -12.4% CBOT rice 1300.50 1.50 0.1% -16.2% EU wheat 170.50 -5.00 -2.9% -18.4% US crude 98.05 -2.22 -2.2% -0.4% Dow Jones 16,590 -290 -1.7% 0.1% Gold 1283.81 -10.89 -0.8% 6.5% Euro/dollar 1.3388 -0.0008 -0.1% -1.9% Dollar Index 81.4600 0.0280 0.0% 1.8% Baltic Freight 755 1 0.1% -66.8%

In U.S. cents, benchmark contracts, except EU wheat (euros) and soymeal (dollars). CBOT wheat, corn and soybeans per bushel, rice per hundredweight, soymeal per ton and soyoil per lb.

(Additional reporting by Michael Hirtzer in Chicago, Naveen Thukral in Singapore and Nigel Hunt in London; Editing by Marguerita Choy and Peter Galloway)