* Pro Farmer sees bigger U.S. corn crop than USDA
* Old-crop U.S. soy supply tightens ahead of bumper harvest
* U.S. output expectations limit gains in deferred soy, corn
* MGEX wheat climbs on concerns about rains hurting crop
(Updates with Pro Farmer crop estimates, analyst comment, closing prices)
CHICAGO, Aug 22 (Reuters) - Nearby U.S. soybean futures climbed to a one-week high on Friday as U.S. processors scrambled to find scarce old-crop supplies, with the start of the next harvest still at least a month away in the Midwest.
MGEX spring wheat futures touched a two-week high amid concerns about excessive rains hurting crops in key production areas of the northern U.S. Plains. New-crop corn and soybeans, meanwhile, crept higher, although expectations for large harvests limited gains.
Pro Farmer, a farm advisory service, projected U.S. 2014 corn production at a record 14.093 billion bushels, based on a yield of 169.3 bushels per acre. The U.S. Department of Agriculture last week estimated the crop at a record high of 14.032 billion bushels on a yield of 167.4 bushels.
Karl Setzer, grain solutions team leader for MaxYield Cooperative in Iowa, said he agreed with Pro Farmer's estimates but thought the harvest may get smaller due to dryness.
"We do have a good crop going, but it's a long way from what I'd call perfect," Setzer said.
New-crop December corn added 0.7 percent to end at $3.71-1/2 a bushel at the Chicago Board of Trade, while November soybeans rose 0.4 percent to $10.42 a bushel.
Nearby September soybeans ended up 2.6 percent at $11.66 a bushel after soaring to a session high of $11.70-1/2, the highest price for a front-month contract on a daily continuation chart since Aug. 14.
The supply situation for soybeans will change dramatically in a few weeks as upcoming harvests will replenish inventories drained by strong demand from domestic processors and importers.
Until then, "processors are falling all over each other to get whatever supplies they can," said Jason Britt, president of Central States Commodities, a brokerage in Kansas City, Missouri.
Pro Farmer forecast U.S. soybean production at 3.812 billion bushels, with an average yield of 45.35 bushels per acre. Last week, the USDA pegged the crop at an all-time high of 3.816 billion bushels, with a yield of 45.4 bushels per acre.
The Pro Farmer tour fuelled expectations among analysts that the USDA will raise its national corn yield forecast.
"The question is simply how high and how soon will the yield number jump," said Kevin Van Trump, chief executive of Farm Direction, a consultancy based in Missouri.
CBOT December wheat advanced 1.2 percent to $5.62-1/4 a bushel after touching a session high of $5.70-1/2, its highest price since Aug. 15. MGEX September wheat added 1.8 percent to $6.26-3/4 a bushel. The contract hit a session high of $6.32 a bushel, its highest prices since Aug. 8.
(Additional reporting by Sybille de La Hamaide in Paris and Naveen Thukral in Singapore; Editing by Marguerita Choy; editing by Michael Urquhart and Matthew Lewis)