The Philippine Institute of Volcanology and Seismology (Phivolcs) issued a lahar advisory on Sunday, Dec. 1, for areas surrounding Mayon Volcano in Albay due to heavy rainfall brought by the shear line.
The shear line is a weather system that forms when cold winds from the northeast monsoon (amihan) meet warm winds from the Pacific Ocean or easterlies, causing rainfall.
Based on the 11 a.m. weather advisory from the Philippine Atmospheric, Geophysical, and Astronomical Services Administration (PAGASA), the shear line is expected to persist, bringing heavy rainfall over parts of Southern Luzon, Bicol Region, and Eastern Visayas.
Phivolcs warned that the heavy rainfall could trigger volcanic sediment flows, or lahars, muddy streamflows and runoff in the drainage areas of Mayon Volcano.
It added that prolonged rains could also lead to non-eruption lahars along various river and stream channels associated with the volcano.
At-risk areas include the Miisi, Binaan, Anoling, Quirangay, Maninila, Masarawag, Muladbucad, Nasisi, Mabinit, Matanag, Bonga, Buyuan, Basud, and Bulawan Channels.
Phivolcs urged local government units and communities in the affected areas to closely monitor weather conditions and implement preemptive measures in response to the ongoing threat from the shear line.