Argentina's agricultural heartland, hit by a drought that is affecting corn and soybean crops, is set for a week of scant rainfall before heavier precipitation towards the end of February, the Buenos Aires grains exchange said on Thursday.

The world's top exporter of processed soy and the no. 2 corn exporter has been suffering from drought linked to the La Niña climate pattern, which has prompted deep cuts in forecasts for harvests of the two grains in recent months, reported Reuters.

Rainfall levels in the coming weeks will be critical. Fears over the drought in Argentina, Paraguay and Brazil have been driving up global grains prices.

The Buenos Aires exchange said that over the next week there will be some rainfall in the farming regions, although mostly moderate amounts between 10 and 25 millimeters, before heavier rains arrive in the last week of the month.

"Towards the middle of outlook, the passage of a storm front will produce general rainfall of varied intensity, with its highest records over the northwest and the center of the agricultural area," the exchange said in a weather report.

It added that the east of the Pampean Region and most of Mesopotamia - some of the zones hardest hit by drought so far - would see "abundant rainfall" of some 25-100 mm, with "pockets of severe storms" from around 24 February.

The drought led the Buenos Aires exchange to cut its 2021/22 soybean harvest estimates by 2 million tonnes to 42 million tonnes and corn by 6 million tonnes to 51 million tonnes. The rival Rosario exchange trimmed its outlook even further.

Esteban Copati, head of agricultural estimates at the Buenos Aires exchange, played down fears of a disaster akin to the drought-hit 2018 season, but said coming rains would be key.

"We will see what happens, depending on how much it rains in the coming weeks. Because sometimes the recovery can be surprising, although we are already in the advanced stages of all crops," he said.

The Buenos Aires exchange said in a separate report that crop conditions had declined over the last week, with expected yields for soy "below the averages of the last five campaigns. Harvesting has started earlier than normal due to the drought.

Early planted corn was the hardest hit by dryness, it added, with late corn looking better: "Despite this, water reserves are increasingly tight, and if this phenomenon persists, it could affect the condition of the crop in future reports."