There are signs that the prevailing La Niña may grow into a “moderate-to-strong” event, with more extreme rainfall occurrences likely to peak in December this year.

rain - La Niña may grow from ‘moderate to strong’ by December

Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) climate monitoring and prediction section chief Ana Liza Solis said that the original La Niña scenario was initially predicted to reach weak-to-moderate intensity but “most forecast models (now) suggest moderate to strong La Niña is likely to persist until February-March-April, 2021.”

“Last month, weak-to-moderate pa pero nakikita natin sa recent conditions ay medyo nasa level na ng moderate (La Niña) dahil ang sea surface temperature (SST) anomaly ay nag-reach na ng negative 1.4. Base sa forecast models ay posible makarating sa moderate-to-strong La Niña. (Last month, it was still weak-to-moderate but we observed that recent conditions is already a moderate La Niña event because the sea surface temperature anomaly has reached negative 1.4. Based on forecast models it is possible that the La Niña could reach moderate-to-strong intensity),” Solis said during the monthly climate outlook forum held on Wednesday.

She explained that a moderate-to-strong La Niña event increases the potential of having extreme rainfall events caused by rain-bearing weather systems, such as northeast monsoon or “amihan,” tail-end of cold front, intertropical convergence zone, low pressure areas, and tropical cyclones.

More rainfall than usual could reach its peak in December, she added, citing that “adverse impacts are highly likely over vulnerable areas and sectors of the country.”

Generally most parts of the country are likely to experience above normal rain fall conditions from November to April except for some areas that may receive below to near normal rainfall, Solis said in her presentation.

“Based on historical record, mas malaki ang potential impact ng La Niña in terms of increased rainfall sa Luzon, particularly MIMAROPA and Bicol, most of Visayas, and eastern section of Mindanao. (Based on historical record, La Niña has greater potential impact in terms of increased rainfall in Luzon, particularly MIMAROPA (Oriental Mindoro, Occidental Mindoro, Marinduque, Romblon, Palawan) and Bicol (Albay, Camarines Norte, Camarines Sur, Catanduanes, Sorsogon, Masbate), most of Visayas, and eastern section of Mindanao,” Solis pointed pointed out.

Citing recent studies, rainfall in Luzon will likely be 35 percent more than the usual, 41 percent higher than average in Visayas, and around 19 percent above average in Mindanao during a La Niña.

Solis said three to seven tropical cyclones may enter or develop within the Philippine area of responsibility from November 2020 to April 2021–one to three in November, two or three in December, and up to one cyclone each in January, February, March, and April.

“Normally, when there is no La Niña, there is only a slim chance of having tropical cyclones from January to April. But we are expecting higher chance of tropical cyclone development in early 2021,” she added.

Solis also noted that tropical cyclone formation happens much closer to the Philippines during a La Niña event.

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