GRAINS-Corn, soy up about 2 pct as export data sparks short-covering

* Weekly U.S. corn, soy export sales totals top expectations

* Rains stall harvest in northwestern Midwest

* Wheat futures end firm; weaker dollar lends support

(Updates with closing U.S. prices) CHICAGO, Sept 20 (Reuters) - U.S. corn futures climbed nearly 2 percent on Thursday, hitting a one-week high as higher-than-expected weekly export sales and prospects for U.S. harvest delays sparked a round of short-covering, analysts said. Soybeans also rose about 2 percent, while wheat futures posted modest gains after a choppy session. Chicago Board of Trade December corn settled up 6-3/4 cents at $3.52-1/2 a bushel after reaching $3.56-1/4, its highest since Sept. 12. November soybeans ended up 20-1/4 cents at $8.50-1/4 a bushel after posting a near-four-week top at $8.55, while December wheat finished up 1-1/2 cents at $5.24. Corn led the way up after the U.S. Department of Agriculture reported weekly export sales of U.S. corn at nearly 1.4 million tonnes, topping a range of trade expectations.

Through its daily reporting system, the USDA also said that private exporters sold an additional 160,020 tonnes of U.S. corn to Mexico. As well, heavy rains in the past day caused localized flooding in parts of the northwest Corn Belt, the Commodity Weather Group said, stalling harvest activity. "It's all about short-covering," said Tom Fritz, a partner with EFG Group in Chicago. "Corn export sales were pretty decent, based on expectations. Soybean sales were decent. You had some really big rain go through," Fritz said, adding: "You couple that with the degree that the spec is short, and it's like, 'We better get some of this covered."' Weekly U.S. soybean export sales topped 900,000 tonnes, surpassing trade expectations and helping buoy the futures market, two days after the most active CBOT soybean contract hit its lowest in 10 years. "Soybeans still have some significant fundamental obstacles with the China trade war, African swine fever (in China's hog herd) and a massive record harvest in front of us. Yet nobody wants to be the last one to recognize a bottom, leading to strong volume in today's action," chief commodities economist Arlan Suderman wrote in a client note. The American Soybean Association will continue to lobby Washington to negotiate a solution to end the escalating trade war between China and the United States, the association's president, John Heisdorffer, said at an industry conference.

CBOT wheat futures closed higher but pared gains in choppy trade after the December contract failed to push through chart resistance at its 200-day moving average. A weaker U.S. dollar lent support, theoretically making U.S. grains more competitive on the global market. The wheat market has been underpinned by crop losses in some major exporting countries, including Australia, and a run of import tenders.

CBOT settlement prices:

Net Pct Volume

Last change change

CBOT wheat WZ8 524.00 1.50 0.3 55436 CBOT corn CZ8 352.50 6.75 1.9 250052 CBOT soybeans SX8 850.25 20.25 2.4 151420 CBOT soymeal SMZ8 314.40 5.20 1.7 58444 CBOT soyoil BOZ8 27.86 0.36 1.3 78154

NOTE: CBOT December wheat and corn and November soybeans shown in cents per bushel, December soymeal in dollars per short ton and December soyoil in cents per lb.

(Reporting by Julie Ingwersen; Additional reporting by Gus Trompiz in Paris and Naveen Thukral in Singapore; Editing by James Dalgleish and Peter Cooney)