(Updates with closing prices)
By Mark Weinraub
CHICAGO, Dec 17 (Reuters) - Chicago Board of Trade soybean and corn futures rose on Thursday, with soybeans soaring past the $12 a bushel mark and hitting a fresh 4-1/2-year high after a strong weekly export report sparked a rally, traders said.
The good demand for U.S. supplies highlighted concerns about potential harvest shortfalls in South America tightening the global supply base as the weather outlook remained uncertain as crops headed toward key development periods.
"We are at the crucial time," said Mark Schultz, chief market analyst at Northstar Commodity. "They better start getting the moisture."
Chicago Board of Trade January soybean futures SF1 settled up 17-1/2 cents at $12.01-1/4 a bushel. CBOT March corn futures CH1 gained 5-1/4 cents to $4.32-1/2.
"This is going to be a big weekend for weather in South America, with the rains over Brazil this week simply not matching their money back guarantees, and the runs slightly drier for Argentina this weekend," Charlie Sernatinger, global head of grain futures at ED&F Man Capital said in a note to clients.
The U.S. Agriculture Department on Thursday morning said that weekly corn export sales totalled 1.935 million tonnes and soybean export sales totalled 1.016 million tonnes. Both topped the high end of trade expectations. wheat futures also rose, breaking through the psychologically important $6 a bushel level on support from improving export prospects due to a weak dollar and Russia's recently announced export tax.
Grain traders said on Thursday they are experiencing delays in obtaining export documents from Russia's customs service. Extra customs checks could be aimed at ensuring that grain exports do not accelerate in the run up to Feb. 15, when the wheat export tax is due to be introduced, the traders said. March soft red winter wheat WH1 ended up 10-1/4 cents at $6.08-3/4.