SAN FERNANDO, Pampanga— As relentless southwest monsoon (Habagat) rains continue to drench Luzon, state scientists have issued a high-level lahar advisory, warning that prolonged heavy rainfall across the Mount Pinatubo landscape could trigger destructive volcanic sediment flows across three provinces.

The Philippine Institute of Volcanology and Seismology (PHIVOLCS), together with PAGASA, said continuous and intense rainfall, even without volcanic activity, could remobilize massive deposits of ash, sand and boulders left by the 1991 eruption and send them rushing down river systems toward populated communities.

“Prolonged and intense rainfall may generate non-eruption lahars on major rivers draining Pinatubo Volcano,” the official advisory stated. “Lahars may begin as channel-confined flows in upper reaches but can transition into destructive muddy streamflows and flash floods in lower-lying areas — with little to no warning.”

Authorities identified four major drainage systems where the lahar threat is considered most severe.

In Zambales, the Sto. Tomas-Marella River System covers San Marcelino, San Narciso and San Felipe, while the Bucao River System covers Santa Fe, Botolan, Cabangan, Iba and Palauig.

In Tarlac, the O’Donnell River System covers Capalongan, Bamban and downstream communities.

In Pampanga, the Pasig-Potrero River System covers Porac and Angeles City and extends to Lubao and Guagua.

Lahars can carry boulders, ash and debris, potentially burying homes and roads. They can also rise rapidly even after rain stops upstream, harden like cement once settled, and be triggered by rainfall far upstream even when there is no rain in the affected community.

Heavy monsoon rains also raise the risk of muddy streamflows and debris flows around Taal Volcano, particularly in Agoncillo, Laurel and Talisay in Batangas, where loose volcanic deposits may be washed down the volcano’s western slopes toward nearby communities.

PHIVOLCS and local disaster officials advised communities in high-risk areas to activate barangay warning systems at the first sign of heavy rain, identify evacuation routes away from river channels and monitor official advisories from DOST-PHIVOLCS, PAGASA and local governments.

Residents were also advised to avoid sleeping in ground-floor rooms in lahar-prone barangays and keep rivers and drainage channels clear of debris, as blockages could worsen the impact of lahars.

The southwest monsoon is forecast to continue bringing rain through 31 August and potentially beyond, keeping the lahar threat elevated.

The Office of Civil Defense has also prepositioned response assets across Zambales, Tarlac and Pampanga in anticipation of possible evacuations.