Camille Naredo

Satellite image with overlayed wind streams illustrating the shear line event on January 16, 2017. Photo courtesy of Olaguera et al. 2025


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shear line

Ariel Rojas

ABS-CBN News

<div><img src="https://od2-image-api.abs-cbn.com/prod/editorImage/1765983834137viber_image_2025-12-17_21-59-03-245.jpg" style="width:960px" class="fr-fic fr-dib" alt="Satellite image with overlayed wind streams illustrating the shear line event on January 16, 2017. Photo courtesy of Olaguera et al. 2025" width="960"/>Satellite image with overlayed wind streams illustrating the shear line event on January 16, 2017. Photo courtesy of Olaguera et al. 2025</div>MANILA (UPDATED) — A team of predominantly Filipino weather scientists developed a physics-based algorithm to detect and monitor shear lines.Shear lines form when the northeast monsoon or Amihan converges with the Easterlies blowing from the Pacific and can trigger heavy to extreme rainfall events over the eastern seaboard of the country.This weather system constantly shifts and may be challenging to spot even with the aid of satellite imagery.&quot;Kung pagbabasehan lang &#x27;yung satellite, kalat-kalat siya na clouds. Hindi talaga siya straight line so kailangan hanapin &#x27;yung area kung saan nagsasalubong &#x27;yung hangin,&quot; lead researcher Dr. Lyndon Mark Olaguera of the Manila Observatory and Ateneo de Manila University explained.(If we base shear line detection on satellite imagery, we&#x27;ll only see cloud clusters. You need to find where the winds converge to identify the exact location.)Unlike tropical cyclones or cold fronts, there are no universally-accepted protocols for detecting shear lines.&quot;Our study is the first to develop an objective index for monitoring and detecting shear lines over the Philippines,&quot; Olaguera said.The research entitled &#x27;An objective method to locate shear lines during the northeast monsoon in the Philippines,&#x27; was published in the Scientific Online Letters on the Atmosphere of the Meteorological Society of Japan in November 2025.Other members of the team include PAGASA weather forecaster Dr. John Manalo, Dr. Jun Matsumoto of the Tokyo Metropolitan University, Dr. Faye Abigail Cruz of the Manila Observatory, and former Ateneo de Manila University president and Manila Observatory Executive Director Fr. Jose Ramon Villarin.The researchers identified that changes in wind patterns, moisture concentrations, and spinning in the lower atmosphere resulting from the convergence of wind systems can help estimate the location of shear lines.Applying these parameters in the detection algorithm for climatological data from 1979 to 2022, shear lines were found to contribute 20 percent of the total extreme rainfall days over the eastern part of the Philippines during the Amihan season.The algorithm was validated and able to depict the forecast position of a shear line that triggered heavy rainfall over Bicol Region and Eastern Visayas on January 9, 2025.&quot;Sinubukan namin &#x27;yung algorithm sa forecast ng isang shear line event noong January 9, 2025 and nalaman namin na applicable siya - na kaya niyang i-detect &#x27;yung shear line na nagpabaha sa Eastern Visayas nung time na &#x27;yun. So puwede siyang magamit for early warning system, monitoring,&quot; Olaguera said.(We tried the algorithm to forecast a shear line event on January 9, 2025. It was able to detect the shear line that caused flooding in Eastern Visayas. It can be used for early warning system and monitoring.)The study also found that shear lines occur more frequently between November and January and are more likely to affect Luzon and Visayas, particularly in December, than Mindanao.<div> </div><div>IMPROVING WEATHER FORECASTING AND EARLY WARNING</div>Olaguera said that the primary application of their algorithm is for weather forecasting and development of early warning systems.&quot;Kapag meron nang algorithm na magagamit sa operational forecasting, mas madali nating ma-de-determine kung saan area uulanin at makaka-experience ng posibleng pagbaha.&quot;(If the algorithm is used in operational forecasting, it would be easier to determine which areas could receive significant rainfall and experience flooding.)He added that the algorithm may be endorsed to PAGASA for operational use.&quot;May MOU (memorandum of understanding) naman kami (Manila Observatory) with PAGASA so puwede naming ibigay ang algorithm sa kanila para mas magamit nila sa operational forecasting.&quot;(There is an existing MOU between MO and PAGASA so we can share the algorithm with them for operational use.)The researchers said the shear line-specific algorithm may also be used for monitoring of other wind convergence systems such as the intertropical convergence zone or ITCZ, especially when combined with artificial intelligence.The term shear line was formally adopted by PAGASA only in 2020 as the agency used to call the weather system the &#x27;tail-end of a cold front&#x27; from 1980s.One notable recent shear line event happened on November 22, 2023.Almost 620 millimeters of rain was dumped in Catarman, Northern Samar in just 24 hours - equivalent to six weeks of rain and more than Ondoy&#x27;s precipitation.The extreme rainfall event spawned life-threatening floods in many areas of the province.Amid the climate change, experts warn that rain-bearing weather systems such as shear lines may bring heavier rainfall, as a warmer atmosphere can hold more moisture and fuel extreme rainfall events.

A team of predominantly Filipino weather scientists developed a physics-based algorithm to detect and monitor shear lines.

A team of predominantly Filipino weather scientists developed a physics-based algorithm to detect and monitor shear lines.

Filipino meteorologists devise algorithm to detect, monitor shear lines

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Filipino meteorologists devise algorithm to detect, monitor shear lines

Ariel Rojas,

ABS-CBN News

Published December 18, 2025 12:27 AM PHT

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Updated December 18, 2025 07:00 PM PHT

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shear line

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