GRAINS-Soybeans surge to 5-month high on S.American crop concern

* Adverse weather hits Brazil, Argentina soybean output

* Wheat gains for 3rd day to test $6 mark, corn also up

(Recasts with U.S. market opening, adds quotes, updates prices, changes byline, changes dateline from PARIS/SINGAPORE) CHICAGO, Feb 18 (Reuters) - U.S. soybean futures surged to a five-month high on Tuesday as adverse weather in South America fuelled concerns about smaller crops in key exporters Brazil and Argentina. Wheat rose for a third consecutive session, underpinned by technical momentum from recent gains and worries that a harsh winter may cut production in the United States. Corn prices firmed on spillover support from rallying soybeans, but abundant supplies following a record-large U.S. harvest anchored the market. "The U.S. grain trade has been behind in reacting to the weather situation in South America," said Rich Nelson, chief strategist with consultancy Allendale Inc. Analysts at Brazilian consultancy AgRural shaved 1.8 million tonnes off their estimate for Brazil's 2013-14 soybean crop on Monday, saying drought in much of the country in recent weeks had reduced yields. AgRural now expects a crop of 87 million tonnes, down from 88.8 million tonnes previously and well below the U.S. Department of Agriculture estimate for 90 million tonnes. "After the decline in AgRural's soybean production estimate over the weekend, we're wondering if we maybe have to start taking a bit off the expectations we've been talking about over the last couple of weeks," Nelson said. In Argentina's main soybean-growing region, weeks of heavy rain have encouraged the spread of caterpillars that are eating into the 2013/14 season's crop yields, a climate expert said on Monday. The less favourable conditions in South America added to concerns about tight U.S. soybean stocks, which have been whittled down by record-strong export demand. Meanwhile, cancellations of U.S. purchases by China and switching of orders toward South American supplies have been below expectations. Chicago Board of Trade March soybeans jumped 22-3/4 cents, or 1.7 percent, to $13.60-1/4 per bushel by 10:30 a.m. CST (1630 GMT), the highest for a spot-month contract since Sept. 19. CBOT March wheat rose 9 cents, or 1.5 percent, to a six-week peak of $6.07-1/2 a bushel. CBOT March corn added 2-1/4 cents, or 0.5 percent, to $4.47-1/2 a bushel.

Prices at 10:35 a.m. CST (1635 GMT)

LAST NET PCT YTD CHG CHG CHG CBOT corn 447.50 2.25 0.5% 6.0% CBOT soy 1360.00 22.50 1.7% 3.6% CBOT meal 456.90 6.90 1.5% 4.4% CBOT soyoil 40.14 0.99 2.5% 3.4% CBOT wheat 607.75 9.25 1.6% 0.4% CBOT rice 1563.00 4.50 0.3% 0.8% EU wheat 199.00 0.75 0.4% -4.8% US crude 101.54 1.24 1.2% 3.2% Dow Jones 16,153 -2 0.0% -2.6% Gold 1321.84 -6.65 -0.5% 9.7% Euro/dollar 1.3754 0.0048 0.4% 0.7% Dollar Index 80.0170 -0.1190 -0.2% 0.0% Baltic Freight 1146 16 1.4% -49.7%

(Additional reporting by Gus Trompiz in Paris, Naveen Thukral in Singapore; Editing by Alden Bentley)