Shear Line and Amihan: Why Parts of Luzon and the Visayas Can See Heavy Rains
Heavy rains and regional weather patterns
Periods of heavy rain over parts of Luzon and the Visayas are often discussed in the same breath as familiar seasonal wind patterns and boundary zones in the atmosphere. While specific local triggers can vary from week to week, the broader setting matters: Asia and the Pacific sit beside some of the world’s most active waters for tropical cyclone formation, and the region repeatedly experiences weather systems capable of producing intense rainfall, flooding, and landslides.
Recent weeks have underscored how disruptive these rain-producing systems can be across South Asia and Southeast Asia. Cyclones in the region brought heavy rain and flooding that reportedly killed at least 1,800 people, displaced more than one million residents, and affected nearly 11 million. Those figures highlight a key reality for communities across the western North Pacific and nearby basins: the threat is not limited to coastal wind damage. Prolonged and intense rainfall can become the dominant hazard, including far inland.
Why the Asia-Pacific region frequently sees cyclone tracks
From year to year, tropical cyclones repeatedly affect countries across Asia and the Pacific because the region often provides the essential ingredients storms need. A cyclone is a very large storm that forms over tropical waters. These storms are called hurricanes in the Atlantic and the eastern Pacific, typhoons in East Asia, and cyclones in the Indian Ocean and the South Pacific—though “cyclone” is also commonly used as a general term for this type of tropical storm.
Globally, there are seven regions that frequently experience tropical cyclones. Historically, the western North Pacific has been the most active area for cyclone formation. The Philippines sits in the middle of what is sometimes called a “typhoon belt,” averaging around 20 typhoons each year. A record of cyclone tracks points to the area east of the Philippines, east of Taiwan, and south of Japan as one of the world’s most active zones, with many “super typhoons” forming there and some seasons exceeding 26 cyclones.
Warm seas: the main fuel for rain and wind
Warm tropical oceans are the foundation of cyclone development. Storms typically form where sea temperatures are above about 26 degrees Celsius down to a depth of around 60 meters. Sebastien Langlade, Head of Operations at the Regional Specialized Meteorological Center La Réunion, described warm sea-surface temperatures as the main “fuel” for cyclone formation.
This “fuel” matters directly for rainfall. Warm seas promote evaporation, adding moisture to the atmosphere. When the atmosphere contains sufficient moisture, rising air can condense into clouds and precipitation. In practical terms, the same heat and moisture that help organize a tropical cyclone can also support widespread, heavy rain—even when the most intense winds are concentrated elsewhere.
Atmospheric conditions that allow storms to organize
Warm water alone does not guarantee a cyclone or a major rain event. The atmosphere must also cooperate. One key factor is how winds change with height. For a developing storm to organize, the direction and speed of winds from the sea surface up to altitudes of about 15–20 kilometers need to be relatively uniform. If winds at different heights vary too sharply, a developing system can be disrupted before it becomes a mature cyclone.
Another factor is latitude. Cyclones generally do not form too close to the equator because the Coriolis force there is too weak to generate the rotating structure needed for a cyclone’s circulation. As a result, cyclones typically form at latitudes greater than about 5 degrees. This basic constraint helps shape where storm formation is most likely and, by extension, which areas more often find themselves along common storm paths.
How a low-pressure system turns into a major rain producer
The development process begins with a low-pressure system that encourages warm, moist air to rise. When clusters of thunderstorms form within a warm low-pressure area, the system can intensify by drawing in more warm, moist air through evaporation from the ocean surface. As this air rises, it cools and produces more clouds, supporting further storm growth.
Langlade described the mechanism in energetic terms: a tropical cyclone can absorb energy from the ocean and convert it into wind and rain. In this view, the cyclone functions like a thermodynamic engine, powered by the heat and moisture available over warm tropical waters. Even for people focused mainly on rainfall impacts, this is a useful way to think about why some systems can sustain long periods of heavy precipitation.
Rainfall, flooding, landslides, and storm surge: the full hazard picture
Strong winds are often the most visible danger, but tropical cyclones can also produce very heavy rainfall, severe flooding, and storm surge. Langlade described storm surge as a rise in sea level accompanied by large waves, which is one reason sea conditions become extremely dangerous when a tropical cyclone reaches high intensity.
The recent impacts described across South Asia and Southeast Asia—heavy rain, flooding, and landslides—illustrate how cyclone hazards extend beyond coastal wind damage. Inland areas can face serious risk when sustained rainfall saturates soils and overwhelms drainage systems. In places with steep terrain or vulnerable slopes, this can raise the likelihood of landslides during prolonged wet periods.
Examples of recent cyclones and why they mattered
Several named systems were highlighted in the recent period, reflecting the region’s repeated exposure to rain-producing cyclones:
Ditwah struck Sri Lanka and triggered landslides and floods described as the worst in the country’s recent history.
Koto affected the Philippines and Vietnam.
Senyar caused flooding and landslides across three countries.
Senyar drew particular attention because it formed in the Strait of Malacca, a narrow body of water between Malaysia and the island of Sumatra. Climatologist Fredolin Tangang—an emeritus professor at Universiti Kebangsaan Malaysia and a former vice-chair of a United Nations IPCC working group—described the phenomenon as highly unusual. According to NASA, it is only the second recorded case of a tropical cyclone forming in the Strait of Malacca.
Earlier in November, the Philippines was also hit by Fung-wong, described as the strongest cyclone to make landfall in the country during 2025. With a diameter reaching about 1,800 kilometers, it influenced 16 of the Philippines’ 18 regions. That scale is a reminder that a single system can have wide geographic reach, spreading rain bands and unstable weather far from the storm’s most intense core.
Understanding cyclone categories and seasons
Storm strength is categorized, and terminology can be confusing. When wind speeds reach roughly 119 kilometers per hour or higher, a storm is officially classified as a tropical cyclone. Based on wind speed, cyclones are then grouped into Category 1, 2, or 3 using the Saffir–Simpson scale, as described in the provided material.
Cyclone seasons also differ by hemisphere. In the Northern Hemisphere, the cyclone season usually runs from June to November. In the Southern Hemisphere, it typically spans November to April. However, storms can still form outside these windows, which is one reason forecasters and disaster managers emphasize readiness beyond a strict calendar.
Why preparedness matters even when the calendar changes
One challenge for communities in cyclone-prone regions is that risk awareness can fade after quiet periods. Langlade stressed that public readiness can decline when there are several years without a major cyclone threat. Yet the region’s track record shows that damaging storms can return, and that the most serious consequences often come from water: heavy rain, flooding, and landslides.
There is currently no known way to stop or weaken a cyclone once it forms. That reality places greater weight on actions that reduce harm. Tangang argued that while typhoons cannot be avoided, their impacts can be reduced through systematic adaptation and through infrastructure such as flood-control measures.
Langlade also emphasized practical steps that can be taken at the start of cyclone season, such as trimming tree branches and clearing drainage channels so floodwater can flow rather than overflow. These measures are straightforward, but they reflect a broader principle: reducing exposure to floodwater is often about ensuring water has somewhere to go.
Climate signals and what scientific assessments suggest
Scientific work continues to examine how cyclone activity may be changing as the climate warms. A study published in the journal Nature during the summer reported that recent global warming patterns have been “shifting the location of cyclone clusters from the western North Pacific to the North Atlantic.”
Tangang emphasized a broader point: science shows clearly that the warmer the Earth becomes, the more extreme weather events can be. In the context of tropical cyclones, warmer oceans associated with human-driven climate change can contribute to stronger storms and higher-category systems.
According to the latest IPCC report referenced in the material, as the world continues to warm, the proportion of very intense Category 4 and 5 cyclones is expected to increase worldwide. Even if the overall number of cyclones does not rise, the impacts could become more severe, including higher wind speeds, larger storm surges, and increased rainfall. Langlade described this outlook as concerning.
What this means for rain-prone areas in Luzon and the Visayas
When heavy rains are forecast over parts of Luzon and the Visayas, it is useful to place the outlook within the region’s larger meteorological context. The western North Pacific is historically the most active cyclone basin, and the Philippines lies in a corridor where storms often form or pass nearby. Even when a cyclone is not directly overhead, the same warm seas and moisture-rich atmosphere that support cyclone development can also contribute to prolonged rain events.
Importantly, the hazards associated with these systems are not limited to wind. The recent regional record—flooding, landslides, and widespread disruption—shows why rainfall forecasting and flood preparedness remain central concerns. Sustained rain can saturate soils, overwhelm drainage, and trigger slope failures, creating dangerous conditions well away from coastlines.
Key takeaways
Asia and the Pacific frequently experience tropical cyclones because warm seas and supportive atmospheric conditions often align in the region.
Warm ocean waters supply heat and moisture, helping storms convert ocean energy into wind and heavy rain.
Rainfall impacts—flooding and landslides—can be as dangerous as wind, including in inland areas.
Storms can form outside typical seasonal windows, reinforcing the need for preparedness beyond a strict calendar.
There is no known way to stop a cyclone once it forms, so adaptation, flood-control measures, and practical readiness steps are essential.
Across Asia and the Pacific, the repeated experience of tropical cyclones points to a consistent set of realities: warm tropical seas can provide the energy storms need; atmospheric conditions influence where and when they form; and the hazards extend from wind damage to flooding, landslides, and storm surge. For communities in rain-prone parts of Luzon and the Visayas, the focus remains on readiness, adaptation, and risk reduction—especially as scientific assessments warn that a warming world can intensify the most severe storms.