(Updates with closing prices, adds analyst quote, Fed decision, ethanol move)
By Mark Weinraub
CHICAGO, Dec 14 (Reuters) - U.S. soybean futures fell to a three-week low on Wednesday as forecasts for rain in dry areas of Argentina's crop belt took out some of the weather premium built into the market, traders said.
"Beans are seeing a little bit of pressure ... amid a favorable weather forecast for Argentina," CHS Hedging said in a note to clients. "Decent demand continues to limit downside."
Wheat and corn futures closed firm, supported by a late round of short-covering by investors looking to unwind bearish bets they had placed on the grains. Wheat futures have risen for five days in a row while corn has notched a four-session winning streak.
The market moves were muted ahead of the conclusion of the Federal Reserve's two-day policy meeting that saw the U.S. central bank raise interest rates for the first time in a year.
Chicago Board of Trade January soybean futures settled down 4-1/4 cents at $10.23-3/4 a bushel. Prices bottomed out at $10.20-3/4 a bushel earlier in the session, the lowest for the most-active contract since falling to $10.17-1/4 on Nov. 23.
The Argentine rain outlook boosted crop prospects from the key exporter. The United States already faces fierce competition for overseas deals from South America, with a huge crop expected in Brazil.
Brazil's Abiove on Wednesday forecast that the country will produce 101.7 million tonnes of soybeans in the 2016/17 crop year, with 58 million tonnes of the crop expected to be exported.
But strong domestic demand limited the sell-off in soybeans.
The National Oilseed Processors Association (NOPA), the largest U.S. trade group for the industry, is expected to report that its members crushed 162.568 million bushels of soybeans in November, up 4.1 percent from a year earlier. If realized, that would be the busiest November ever and the seventh-largest monthly crush on record.
CBOT March soft red winter wheat ended up 1/2 cent at $4.18 a bushel.
K.C. March hard red winter wheat futures settled up 4-3/4 cents at $4.20 a bushel, with concerns about sub-freezing temperatures damaging the dormant crop lending additional support.
CBOT March corn was 1 cent higher at $3.62 a bushel.
Ethanol futures tumbled on Wednesday after weekly U.S. government data showed that ethanol output hit a record high and stocks also rose. Fuelled by strong margins, ethanol plants upped their production ETN-FUELUS-EIA to 1.040 million barrels per day last week, surpassing record levels seen in August, EIA data showed. (Additional Reporting by Naveen Thukral in Singapore and Sybille de La Hamaide in Paris; Editing by Will Dunham and James Dalgleish)