* Rain forecast for Brazil from Sunday to Tuesday

* Wheat drops 6.5% from this week's high

* Corn futures off 1.3%

(Recasts with US trading, adds analyst comment, updates prices, changes byline/dateline, previous PARIS/CANBERRA)

CHICAGO, Nov 17 (Reuters) - Chicago grain and soybean futures declined on Friday as the prospect of rain in drought-stricken Brazil eased crop concerns, while traders exited long positions heading into a shortened Thanksgiving trading week.

Soybeans, corn and wheat all dipped more than 1%.

The most-active soybean contract on the Chicago Board of Trade (CBOT) fell 17-1/4 cents to $13.43 a bushel by 11:45 a.m. CST (1745 GMT) and set a session low of $13.39-1/2, the lowest price this week.

Soybean prices increased after nearing a two-year low in mid-October amid Chinese demand and concerns about hot, dry weather hurting crops in northern and central Brazil.

Price-pressuring forecasts for Brazilian rains have now eased concerns over how the weather in key planting areas could hurt production in the top exporting nation. Futures prices pulled back after reaching an August high on Wednesday.

"It's all based on the prospect of rain," said Dan Basse, president of Chicago-based consultancy AgResource, noting there is rain in the forecast from Sunday to Tuesday. "There's profit taking. A lot of traders are averting risk."

The showers next week will briefly ease stress from heat and dryness in the northern 40% of Brazil's soybean area, Commodity Weather Group said. Northern Brazil is then expected to dry out again, the firm said.

Basse noted the tight presidential election in Argentina, a major global agriculture player, added pressure on the market. Traders getting out of long positions heading into the Thanksgiving holiday on Thursday also weighed on prices.

CBOT front-month December wheat futures were down 3/4-cent at $5.52-3/4 a bushel after falling to $5.47, a 6.5% drop from this week's high.

Wheat prices were near September's three-year low of $5.40 as Russia, wrapping up a second consecutive huge harvest, continues to export huge amounts of cheap grain. Wheat was also pressured by this week's U.S. Department of Agriculture (USDA) report that showed weekly U.S. export sales were below expectations. CBOT December corn was 6 cents lower at $4.68-3/4 per bushel. (Additional reporting by Peter Hobson in Canberra and Sybille de La Hamaide in Paris; Editing by Alexander Smith)