CHICAGO, June 23 (Reuters) - Chicago Board of Trade corn and soybean futures fell to one-week lows on Friday, pressured by forecasts for beneficial rains in the U.S. Midwest, traders said.

Wheat futures also were lower, with traders locking in profits from a rally that has pushed prices higher for five weeks in a row.

Signs of poor demand, highlighted by lackluster export data and weakness in the cash market, added to the bearish tone hanging over grains futures on Friday.

The latest weather projections pointed to significant rain in parts of the Midwest in the week ahead, which analysts said could bolster the condition of crops before key growth stages this summer.

"Trade (is) continuing to ease overbought conditions as we watch forecasts evolve short term with some relief expected this weekend, and into later next week," brokerage FuturesOne said in a note to clients.

At 10:54 a.m. CDT (1554 GMT), CBOT December corn futures were down 29-3/4 cents at $5.91 a bushel and CBOT November soybean futures were off 34 cents at $13.05-1/2 a bushel.

"These (dry) conditions have not ... irreversibly impacted corn and soybean yields until now. Everything will depend on the weather evolution in the next three weeks," consultancy Agritel said.

CBOT soft red winter wheat futures for September delivery were down 10 cents at $7.42-3/4 a bushel. The contract has jumped 20.4% in the last five weeks.

The U.S. Agriculture Department said on Friday morning that weekly export sales of corn totaled just 83,100 metric tons, near the low end of market estimates that ranged from zero to 800,000 metric tons.

Wheat export sales were reported at 123,900 metric tons and soybean export sales at 626,300 metric tons. That compares with trade expectations of 100,000 to 400,000 metric tons for wheat and 100,000 to 900,000 metric tons for corn.

On the cash market front, spot basis bids for both corn and soybeans have tumbled at processors and elevators this week as a spate of heavy farmer selling has satisfied demand at grain terminals around the Midwest. (Additional reporting by Gus Trompiz in Paris and Matthew Chye in Singapore; Editing by Rashmi Aich, Sohini Goswami, Shailesh Kuber and David Evans)