Updates throughout with market moves, changes dateline from PARIS/BEIJING

By P.J. Huffstutter

CHICAGO, June 25 (Reuters) - Chicago Board of Trade corn futures turned higher on Thursday on a key technical reversal, as oil prices turned higher, the U.S. dollar eased and traders focused on forecasts of a heat wave and high-pressure weather system set to hit the U.S. Corn Belt this weekend.

The move ended a four-session slide in corn contracts, which had slumped on fund liquidation, falling crude oil prices and a rising U.S. dollar. FRX/ O/R

CBOT soybean futures also surged on weather concerns as traders added risk back into the market, market analysts said, after funds aggressively liquidated their corn and soybean positions and investors pushed to exit expiring futures contracts and roll positions forward.

Wheat futures were steady to slightly higher, as traders focused on potential heat wave damage in Western Europe and concern about wheat harvest issues in the Northern Hemisphere.

The most-active corn contract on the Chicago Board of Trade (CBOT) Cv1 was up 1.29% at $4.12-1/4 a bushel at 12:27 p.m. CDT (1727 GMT). CBOT soybeans Sv1 was 1.48% higher at $11.51-3/4 a bushel, while CBOT wheat Wv1 ticked up 0.29% at $5.97-3/4 a bushel.

The latest U.S. Department of Agriculture crop conditions report showed that two-thirds of the U.S. corn and soybean crop was in good to excellent condition, reflecting mostly favorable Midwestern growing conditions. Still, traders said, their attention on Thursday shifted to U.S. weather patterns that are coming in the next week or so.

The National Weather Service is forecasting temperatures that could reach 100 degrees Fahrenheit this weekend as far north as the upper Midwest and as far east as the Carolinas. Hotter-than-normal weather is expected from the Plains to the Atlantic Coast through July 4.

"The catalyst in the market is weather," said Don Roose, president of U.S. Commodities.

Part of the concern is the timing of a heat wave, Roose said. In parts of the Upper Midwest and Plains, corn and soybean roots may still be shallow, and therefore more sensitive to being damaged by wide swings in temperature, especially if nights stay hot and the plants don't get a chance to rest. The other concern is how long such weather may last, Roose said.

"A lot of the trade right now thinks this heat we're supposed to see will be gone very quickly," Roose said. "But I've been here a while, and I'll tell you this: Once these high pressure systems show up, they either stay - or they can leave and come back."

Meanwhile, heavy rain in recent weeks has eased some of the drought areas in the lower Midwest, but much of Iowa and central Illinois and Indiana are seeing pretty wet conditions, according to the National Weather Service and USDA.

Weather forecasters also are calling for up to 4 more inches (10 cm) of rain to fall in the central Plains and Ohio Valley next week. Excessive moisture can harm corn and soybean plants by suffocating roots and creating environments where fungal plant diseases can thrive.

In parts of the South and Southeast, home to the nation's more fringe corn and soybean production acreage, volatile weather patterns have shifted some states from serious drought to outright flooding.

Investors also are anxiously waiting for the USDA planting and stocks data, due on June 30. It is one of the most widely followed reports and will offer insight into how much grain farmers may still have in storage on their farms.