* Traders monitor forecasts for Brazil rains

* USDA reports more US soybean export sales

* US backs ethanol sector on aviation fuel tax credits

CHICAGO, Dec 15 (Reuters) - Chicago Board of Trade soybean futures sagged on Friday but were poised for their first weekly gain in 1-1/2 months, underpinned by solid demand for U.S. supplies and concerns that dry weather will reduce yields in top-exporter Brazil.

The most-active soybean contract traded 2-1/2 cents lower at $13.11-1/2 a bushel by 10:20 a.m. CST (1620 GMT). Wheat rose 5 cents to $6.20-3/4 a bushel, while corn was flat at $4.79-1/4 a bushel.

Soybeans have gained around 0.6% so far this week, in what would be the market's first weekly increase since early November. Wheat has lost about 1.7% this week and corn is down 1.3%.

Strong demand for U.S. cargoes and dryness threatening Brazilian production have underpinned CBOT soy, but there are forecasts for beneficial showers in Brazil.

Commodity Weather Group said forecasting models show strong support for rains to temporarily ease heat and dryness in northern Brazil starting late next week.

Traders are watching forecasts closely as crop losses due the poor weather in Brazil could boost export demand for U.S. soy on the global market.

"We're dependent on South American weather, no doubt," said Don Roose, president of brokerage U.S. Commodities.

The U.S. Department of Agriculture said in a daily reporting system that exporters sold 447,500 metric tons of U.S. soybeans to unknown destinations and another 134,000 metric tons to China. It was the eighth consecutive session in which the USDA announced a daily soybean sale.

In other news, the Biden administration said it will recognize a methodology favored by the ethanol industry in guidance to companies looking to claim tax credits for sustainable aviation fuel, a pivotal win for the politically powerful U.S. corn lobby.

The decision has helped underpin CBOT corn, though the market is still lacking direction, brokers said.

(Reporting by Tom Polansek in Chicago. Additional reporting by Naveen Thukral; Editing by Sherry Jacob-Phillips, Subhranshu Sahu and Chizu Nomiyama)