* Weak dollar helps to support grain and oilseed prices
* Weather risks for soy in August
CHICAGO, Aug 4 (Reuters) - Chicago soy and wheat futures rose sharply on Thursday, rebounding from wheat's six-month low on signs of a pick-up in demand following the recent decline in prices and forecasts for hot weather in the United States.
Corn prices also regained ground after falling earlier in the week.
"It started in the soybean and spring wheat market. That tells me it's a weather story," said Arlan Suderman, chief commodities economist at StoneX.
The most-active wheat contract on the Chicago Board of Trade (CBOT) closed up 2.4% at $7.81-3/4 a bushel after dropping to its lowest since early February at $7.52 a bushel on Wednesday. CBOT soybeans rose 3.6% to $14.19 a bushel.
Soybeans in key growing areas of the U.S. Midwest will likely face more hot and dry conditions during August, their critical growing month, that could threaten harvest yields, according to weather forecasts.
Dealers noted the market was keeping a close watch on developments in Ukraine as grain begins to move out of its seaports for the first time since February, as major challenges remain.
A Turkish bulk carrier is expected to arrive in the Ukrainian Black Sea port of Chornomorsk on Friday and will be the first vessel to arrive at a Ukrainian port during Russia's invasion, a spokesperson for the regional administration of Odesa said.
Weather service Maxar in a report on Thursday said its 6-10 outlook indicated drier weather in the central and northeastern Midwest although rains in the Delta and southeastern Midwest should improve moisture for corn and soybean crops there.
U.S. weekly wheat net export sales for the new marketing year for the week ending July 28 were 249,900 tonnes and U.S. soybean net export sales were 410,600 tonnes, in line with expectations. (Additional Reporting by Gus Trompiz in Paris, Nigel Hunt in London and Naveen Thukral in Singapore; Editing by Subhranshu Sahu, Rashmi Aich and David Evans)
