(Updates CBOT context, adds comment from Buenos Aires Grains Exchange)
BUENOS AIRES, March 26 (Reuters) - Rains expected to relieve Argentina's drought-hit soy and corn crops failed to materialize over the weekend, all but ending hope that yields might recover from four months of unrelenting sun with more heat and dryness expected over the days ahead.
U.S. soybean futures touched a one-week high on Monday after the disappointing weather increased concern that dryness in the world's No. 3 exporter would tighten global supplies. It was not the first time forecasters got it wrong since drought descended on the normally fertile Pampas grains belt in November.
"The weather differed from the forecasts again," said Esteban Copati, the Buenos Aires Grains Exchange's chief analyst. "The storm front dodged most of the agricultural region and even the areas that registered rainfall accumulated amounts well below what was predicted by the different forecast models."
Argentina is the world's biggest supplier of soymeal livestock feed, used to fatten pigs and cattle from Europe to Asia. The drought has put upward pressure on food prices, making it harder for poor countries to feed themselves.
"We had a few showers this weekend, but none of them were important," said Francisco Abello, a partner in the TraulenCO SA farm management company. "Even if it rains going forward, yields are not going to change much. Most of our soybeans are done."
As of Monday, Argentina looked increasingly hot and dry over the 10 days ahead, said Isaac Hankes, a weather research analyst at Thomson Reuters' Lanworth commodities and weather forecaster.
"The heat won't fully kick in until April 6 to the 10th, but it could be intense and dryness is likely throughout the 10-day forecast," Hankes added.
Soy crop estimates started the 2017-18 season in the 55 million tonne range, but have been slashed to around 40 million.
"YIELDS ARE FIXED"
Meteorologists had projected weekend rains of one to two centimeters (0.4-0.8 inch) in key farm areas of Cordoba, Santa Fe and Entre Rios provinces. But the showers never came.
"No matter how much it rains now, it won't help. Yields are fixed," Sofia Corina, a crop analyst at the Rosario grains exchange, said on Monday.
"The first soy to be harvested shows a lot of variability from two to five tonnes per hectare. Soy that was planted later in the season is more affected by the drought, and a lot of those fields will not be harvested at all," Corina said.
The first corn to be harvested has shown better-than-expected yields but later-planted corn was harder hit by the dryness. Farmers were expected not to even bother trying to harvest later-planted corn fields, Corina added.
The scant rains that fell on parts of the Pampas did little or nothing to relieve parched soy and corn fields. The Southern Hemisphere autumn harvesting season started last week.
"We had practically no rain on our land over the weekend," said Pedro Vigneau, who operates a 1,400-hectare (3,500-acre) farm in the central Buenos Aires district of Carlos Casares.
Early planted soy benefited from good ground moisture at the start of the season, Vigneau said. But as the harvest continues, he said yields will fall as later crops, planted under drier conditions, are brought in.
This month the Rosario exchange slashed its soy crop forecast to 40 million tonnes from a previous 46.5 million while cutting its corn estimate to 32 million tonnes from 35 million.
The Buenos Aires Grains Exchange meanwhile cut its soybean harvest estimate to 39.5 million tonnes from 42 million and reduced its corn crop forecast to 32 million tonnes from 34 million tonnes. (Reporting by Hugh Bronstein Editing by Jeffrey Benkoe and Marguerita Choy)