A new study has found that tropical cyclones forming far from the Philippines often pose a greater flood threat than those that directly hit the country during the southwest monsoon or Habagat season.
Researchers discovered that while typhoons that make landfall contribute only 15.4 percent of the rain during the Habagat months of July to September, those that remain at sea but enhance the monsoon contribute an average of 33.1 percent—more than double. Over half of the rainfall during the season still comes from the monsoon itself, unaided by any cyclone.
The findings, based on 62 years of weather data, challenge the common belief that only landfalling storms cause severe flooding. Scientists from Ateneo de Manila University, Manila Observatory, PAGASA, and Japanese institutions analyzed records from 1961 to 2022, focusing on how distant cyclones fuel heavier rains by pulling in more moisture to the monsoon.
One of the starkest examples was Typhoon Gaemi (Super Typhoon Carina) in July 2024. Despite never making landfall, the storm intensified Habagat so severely that Quezon City recorded a month’s worth of rainfall in a single day. The resulting floods across Luzon left 48 people dead and caused over ₱8 billion in damages.
The study also revealed that cyclones forming farther from the Philippines tend to steer northeast of Luzon and have a stronger impact on the southwest monsoon. In contrast, nearby cyclones often travel westward quickly, limiting their influence.
In the rainiest years on record—1962, 1972, 2012, and 2018—non-landfalling storms were responsible for as much as 41.5 percent of the monsoon rainfall. These storms, though distant, had devastating consequences on flood-prone areas like Metro Manila and the western coasts.
Researchers are now urging disaster officials to look beyond a typhoon’s track and consider its interaction with the monsoon.
