Le Dinh Quyet, deputy head of the center's forecasting unit, said southern Vietnam, including Ho Chi Minh City and the Mekong Delta, often entered the rainy season from around May 10-15. But due to the La Nina phenomenon, this year's rainy season may commence as soon as early May, or even late April, local media reported.

Until the end of June, sunny weather should be expected in southern regions during the day, with temperatures ranging between 23-24 degrees Celsius. Heavy rains should be expected around dusk, Quyet said.

In August, as the effects of La Nina subside, the weather would return to a more neutral state, he said.

Earlier rainy seasons have been common in the past few decades, but not to this year's extent, he said.

La Nina is a coupled ocean-atmosphere phenomenon that lowers the sea surface temperature across the eastern equatorial part of the central Pacific Ocean and causes extensive weather effects across the globe.

While Vietnam has avoided its impacts for around a decade, La Nina has proven unpredictable this year, evident by the fact that frequent, widespread and long-lasting rain has been detected in southern Vietnam during early April. Such rains have become heavier over time and lead to lower temperatures across the entire region.

Weather expert Le Thi Xuan Lan said an earlier rainy season would help southern Vietnam enjoy a cooler summer, but the impacts of La Nina means the season would also be shorter than in previous years. As such, southern Vietnam should prepare to store freshwater for possible droughts, she said.