The shear line has been dominating national and local weather reports in recent days as the Philippine Atmospheric, Geophysical and Astronomical Services Administration (Pagasa) predicts rainfall, thunderstorms, flooding, and even landslides in various parts of the country. 

In a forecast report released at 4 a.m. on Friday, Feb. 20, Pagasa said the shear line could affect the Mimaropa, Bicol, Northern Mindanao, Caraga, Zamboanga Peninsula regions, and the provinces of Quezon, and Davao Oriental.

Provinces in these regions might experience “moderate to heavy rains,” possibly triggering flash floods or landslides, Pagasa said. Hours earlier, it said provinces in southern Luzon and the Visayas could expect “heavy to intense rainfall” (Sorsogon, Albay, Northern Samar, Eastern Samar, Southern Leyte, Dinagat Islands, and Surigao del Norte).

Although many are familiar with the term, understanding how and why a shear line occurs can help people take necessary precautions. Recently, it actually brought heavy rains that spawned floods in the eastern and southern provinces of Luzon. 

According to Pagasa, a shear line is a weather phenomenon that occurs when two winds “collide”—the northerly east wind flow (also known as the “amihan”) and the easterlies. 

“The amihan occurs due to high pressure in parts of China, Siberia, and Mongolia, bringing cold and dry air. The easterlies, on the other hand, bring warm and moist air. When these two air masses collide, clouds form, resulting in heavy rainfall,” said Dan Dominic Triumfante, weather forecaster of the agency’s Southern Luzon office.

Triumfante said that shear line-induced rainfall could last up to four days but is not always continuous. This usually occurs when the pressure of the northeast monsoon is high, typically during the months of October to February, and sometimes reaching the first week of March.

Pagasa said in its latest bulletin that moderate to heavy rainfall was predicted up to Saturday, Feb. 22, in the provinces of Oriental Mindoro, Romblon, Marinduque, Palawan, Quezon, Camarines Norte, Camarines Sur, Catanduanes, Albay, Sorsogon, Masbate, Aklan, Capiz, Northern Samar, Eastern Samar, Samar, Biliran, Leyte, Southern Leyte, Dinagat Islands, and Surigao del Norte.

In Palawan, it said, rainfall “may be higher in mountainous and elevated areas,” and impacts in some areas may be worsened by significant rainfall earlier.

Last Thursday, Feb. 19, Pagasa observed that the shear line was affecting the eastern sections of northern and central Luzon (mainland Cagayan, Isabela, Quirino, and Aurora) with possible flash floods or landslides due to moderate to heavy rains.

The agency raised similar chances in the Cordillera Administrative Region and the rest of Cagayan Valley, Metro Manila, the rest of Calabarzon, and Aurora province this time brought about by the “amihan.” 

In Mindanao, rains induced by the easterlies could occur in Mindanao, with “moderate and at times heavy rains” in Basilan, Sulu, and Tawi-Tawi.

Another weather phenomenon noted three days ago was the Intertropical Convergence Zone, a zone of northeast and southeast trade winds that converge near the equator. On Feb. 18, it was  forecast to cause moderate to heavy rains that could trigger flash floods or landslides in the Zamboanga Peninsula, Sultan Kudarat, Sarangani, Basilan, Sulu and Tawi-Tawi.

Pagasa said its heavy rainfall warning is based on forecasts for the next three hours and applies only to the municipal level. Its weather advisory, meanwhile, covers 24 hours but lacks the accuracy of a rainfall warning and is issued only at the provincial level.

While the advisory provides “a broader outlook on potential weather disturbances,” the agency said it  “should not be used as a basis for class suspensions or other official decisions.” —WITH A REPORT FROM ANTONETTE ALZAGA 

For other information about the weather, log on to pagasa.dost.gov.ph or bagong.pagasa.dost.gov.ph.