As the country shifts from dry to wet conditions, the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) has explained the key factors that determine when the rainy season officially begins.

PAGASA weather specialist Benison Estareja said rainfall and wind patterns are the two main parameters used to assess the arrival of the rainy season.

Estareja explained that for the rainy season to be declared, at least seven of the 13 monitoring stations across the western parts of Luzon and the Visayas must report five consecutive days of rain, with each station accumulating a minimum of 25 millimeters of rainfall during that period. 

“Twenty-five millimeters is roughly equivalent to pouring two medium-sized buckets of water over a one-square-meter area,” he said.

In addition to rainfall, PAGASA also closely monitors wind patterns. 

Estareja said the prevailing winds must shift from easterlies (winds from the east) to winds from the west or southwest, which signals the onset of the southwest monsoon, locally known as “habagat.”

Once both the rainfall and wind criteria are met, he said PAGASA will announce the official start of the rainy season, which usually occurs between the second half of May and the first half of June.

4-day weather outlook

From Friday to Monday, May 16 to 19, Estareja said scattered rain showers and thunderstorms are expected to persist in Mindanao, Palawan, Bohol, Cebu, Siquijor, Negros Oriental, and Negros Occidental due to the Intertropical Convergence Zone (ITCZ) and a potential low pressure area (LPA) southeast of Mindanao.

The ITCZ, where winds from the northern and southern hemispheres converge, can trigger flash floods and landslides in affected areas.

Meanwhile, the easterlies, or warm winds from the Pacific Ocean, will continue to bring warm conditions to the rest of the country.

Estareja explained that the easterlies also carry moisture, which results in a high chance of afternoon rain showers and thunderstorms.