Corn, soybeans, and wheat trended higher in overnight trading, and that strength is expected to spill over into Friday's open on the CME Group floor.

Early calls for the grains are corn 6-8 cents higher, wheat 5-7 higher and soybeans 3-5 higher, according to private sources.

In the overnight trade, March corn was 6 1/2 cents higher at $6.44 1/2, January soybeans were 3 3/4 higher at $11.91 1/4 and March wheat was 5 1/2 higher at $6.50 3/4, according to Barchart.com.

USDA released export sales numbers for the grains Friday morning, showing higher-than-expected sales of U.S. corn to Japan, China and Mexico. Overall, U.S. soybean exports were up 1% from the previous week and 11% higher than a year ago.

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There's also fresh talk Friday of returning crop-stressing weather in South America, after reports of sporadic rainfall there in the last 2 days. Though more rain is expected in the short term, the longer-term outlook isn't as friendly.

"During the past 24 hours some beneficial rainfall did fall in sections of Mato Grosso do Sul, western Sao Paulo, and far northern Parana. Rain chances will increase in southern Brazil and eastern Paraguay over the next few days, with some beneficial amounts forecast through the weekend," according to Freese-Notis Weather, Inc., on Friday morning. "Drier weather should return to these areas early to mid next week and all of these areas will then be drier than normal for the 6-10 day period."