CHICAGO, July 28 (Reuters) - U.S. wheat futures rose more than 2% on Wednesday on increasing concerns over U.S. supplies of top-quality spring wheat after early results from an annual crop tour showed the effects of drought in the northern Plains, traders said.

Corn and soybean futures also firmed, but trade was choppy as brokers weighed fears of declining crop ratings against forecasts for cooler temperatures next week and sluggish export demand for U.S. supplies.

Chicago Board of Trade September wheat settled up 14-1/4 cents at $6.88-3/4 per bushel. Minneapolis Grain Exchange September spring wheat futures were back above $9 a bushel, rising 25-1/2 cents to $9.03-3/4.

CBOT benchmark December corn ended 2-3/4 cents higher at $5.49 a bushel and November soybeans finished up 1-1/2 cents at $13.61 a bushel.

Wheat rose after scouts on an annual crop tour said spring wheat yield potential in portions of North Dakota, by far the country's top producer of the food grain, was well below average due to a severe drought.

Tour scouts on Tuesday projected an average spring wheat yield for southern and east central North Dakota at 29.5 bushels per acre on the first day of a three-day tour, below the tour's five-year average of 43.3.

The drought extends into South Dakota and Montana as well as portions of Canada.

"Rains are expected to pick up a bit in South Dakota on Friday, but ... will be too late to improve conditions for spring wheat, as growth is finishing up," space technology company Maxar said in a daily weather note.

Corn and soybean futures ended higher after a choppy session as market players awaited fresh direction. Dry conditions affecting portions of the Midwest crop belt lent support, with soybeans approaching their key pod-setting phase next month.

"U.S soybean crops are on the cusp of their crop crucible with August almost upon us, and so Midwest weather starts to take on greater importance," said Tobin Gorey, director of agricultural strategy, Commonwealth Bank of Australia.

Meanwhile, in Brazil, freezing temperatures on Wednesday and Thursday will hit crops like corn and wheat in the country's southern and southeastern states, according to government food supply and statistics agency Conab. (Reporting by Julie Ingwersen in Chicago Additional reporting by Colin Packham in Canberra and Sybille de La Hamaide in Paris; Editing by David Evans, Matthew Lewis and Sandra Maler)