GRAINS-U.S. wheat retreats from five-week high; corn, soy also down

* Stronger dollar weighs on U.S. wheat, corn and soybeans

* Higher Russian prices boost U.S. wheat export outlook (Adds quote, updates prices)

LONDON, Jan 24 (Reuters) - Chicago wheat futures slipped from a five-week high on Thursday as a stronger dollar helped halt an advance driven by improving prospects for U.S. exports.

The most-active wheat contract on the Chicago Board of Trade was down 0.3 percent at $5.24-1/2 a bushel at 1119 GMT, haven risen to $5.29, its highest since Dec. 19, 2018.

The wheat market is being buoyed by tightening supplies and rising prices in Russia and Ukraine after brisk early-season shipments by the Black Sea exporting countries.

"Russian prices are rising, which is making Black Sea wheat less attractive for importers," said Phin Ziebell, agribusiness economist at National Australia Bank.

"This could provide an opportunity for U.S. wheat to make its way into Asia and the Middle East."

The pace of Russian wheat exports so far this season has exceeded expectations and dealers were looking to an International Grains Council monthly report later on Thursday for further guidance.

"Russias wheat exports are likely to be upwardly revised (by the IGC) given that they have already reached almost the level predicted by the IGC for the crop year as a whole," Commerzbank said in a note.

Dealers said the lack of any guidance from the U.S. Department of Agriculture due to a partial government shutdown would heighten interest in the IGC data.

A weak euro helped to underpin prices in Europe, with March milling wheat futures on Paris-based Euronext up 0.2 percent at 206.25 euros a tonne.

The euro fell on Thursday ahead of a European Central Bank meeting at which policymakers may express caution about slowing economic growth.

Chicago soybeans fell 0.2 percent to $9.13-1/2 a bushel, while Chicago corn slipped 0.1 percent to $3.78-1/4 a bushel.

Forecasters have scaled back estimates for Brazil's soybean harvest due to drought, and hot weather was expected to keep some regions dry.

The average estimate of Brazil's 2018-19 soybean production in a Reuters poll of analysts was 117.06 million tonnes, down from November's poll average of 120.8 million.

Looking ahead to U.S. plantings for 2019, private analytics firm IEG Vantage, formerly known as Informa Economics IEG, projected farmers would plant more corn and less soybean and wheat compared to 2018.

The firm put 2019 soybean plantings at 86.2 million acres, up 1.1 million acres from its mid-December forecast, but down nearly 3 million acres from 2018.

IEG projected U.S. 2019 corn plantings at 91.5 million acres, up from the 89.1 million acres in 2018. The firm put U.S. wheat seedings for 2019 at 47.163 million acres, compared with the 47.8 million a year earlier. (Additional reporting by Naveen Thukral in Singapore; Editing by Dale Hudson)