The Philippines is on the verge of entering the rainy season, as signs point toward the imminent arrival of the Southwest Monsoon, locally known as the Habagat. While the exact date of onset will be officially declared by PAGASA, atmospheric conditions this week suggest that change is already underway.
The broader monsoon circulation appears to be establishing itself across the region. The shift is being influenced by a developing low-pressure area (LPA) in the South China Sea, changes in the Western Pacific Subtropical High, and a deepening low over southern Japan. All of these factors are contributing to a reversal of winds—marking the hallmark of monsoonal transition.
“Monsoon doesn’t necessarily mean constant rain,” said Speta. “It’s more about the change in wind direction—from easterlies to southwesterlies—and the moisture those winds carry in.”
A Broader Regional Pattern
Across Okinawa, Taiwan, and even parts of Myanmar and Thailand, the seasonal southwest flow has already set in. These areas have seen increased rainfall and humid conditions consistent with the monsoon. The Meiyu-Baiu front, which typically sets up across East Asia in late May and early June, is already active.
As a deep low pressure system crosses southern Japan later this week, it will help draw in more moisture-laden air from the southwest—effectively accelerating the onset of the monsoon across the West Philippine Sea.
Meanwhile, the Western Pacific High, which typically drives easterly trade winds across the region, is forecast to shift slightly eastward. This change creates an opening for a southwest monsoonal surge to sweep across the Philippine archipelago, especially the western sections of Luzon.
What to Expect in the Philippines
The impacts will not be felt evenly across the country.
“Not everyone gets heavy rain right away,” Speta noted. “But the pattern change is what matters—it sets the stage for more frequent and widespread rainfall.”
By Thursday through the weekend, the western coast of Luzon—including Ilocos Norte, La Union, Pangasinan, and Metro Manila—is likely to see an increase in rainfall. Totals could reach 200 to 300 millimeters in some areas over a five-day span.
This is not the hit-or-miss convective activity that typically marks pre-monsoon weather. Instead, it’s a more organized and persistent rain regime, driven by broad-scale wind shifts and deep moisture transport from the Indian Ocean.
“We don’t need a named tropical storm to get serious rainfall,” said Speta. “The monsoon itself can be just as disruptive, especially when moisture pools along the mountains or coastal zones.”
Rainfall Warnings and Flooding Potential
It’s important to understand how local warning systems work during this kind of setup. In the Philippines, PAGASA issues yellow, orange, and red rainfall warnings—a system separate from typhoon signal warnings.
These alerts are based on short-term rainfall rates and are crucial for identifying flood threats, particularly in urban and low-lying areas.
“Always take those red rainfall warnings seriously,” Speta advised. “They mean flooding is either happening or imminent.”
Heavy downpours over a short period can overwhelm drainage systems and saturate the ground. “The soil acts like a sponge,” he said. “Once it’s full, any additional rainfall just runs off, and that’s when you see water levels rise quickly.”
This scenario is especially concerning in areas like Baguio City and the Cordillera region, where terrain can compound flooding risks and trigger landslides.
LPA West of the Philippines: Low Cyclone Risk, High Moisture Impact
A low-pressure area west of Luzon is part of the overall monsoonal shift. While forecast models suggest it has a low chance of developing into a tropical cyclone, its role in steering moisture remains significant.
Even if the LPA intensifies slightly, any development would likely be short-lived due to its interaction with strong monsoonal flow. Most of its moisture will simply be pulled northeastward, feeding rainfall over Luzon, Taiwan, and even parts of southern Japan.
“This LPA isn’t a storm threat right now,” said Speta. “But it could supercharge the rain associated with the monsoon, especially along the west coast.”
Historical Context: Right on Schedule
The timing of this year’s rainy season onset aligns closely with historical norms.
In 2021, the onset occurred on June 4. 2022 saw an unusually early start on May 18. In 2023 and 2024, the monsoon arrived around late May to early June, and this year looks to follow that same window.
Climatology suggests that the period between May 28 and June 2 is often prime for monsoonal establishment in the Philippines.
“I’d be surprised if PAGASA doesn’t declare the rainy season within the next few days,” Speta said. “Everything points to that transition already taking place.”
Monitoring the Transition
Weather watchers are encouraged to stay informed throughout the week as changes accelerate. The evolving pattern won’t just affect rainfall—it could also impact sea conditions, air travel, and outdoor activities.
As always, official alerts from PAGASA, Japan Meteorological Agency, and Taiwan’s Central Weather Bureau should be monitored closely for updated warnings and advisories.
Speta plans to issue further updates as the week progresses, including new graphics and breakdowns of model forecasts.
A Personal Note: Remembering Tomoe the Weather Kitty
For those who’ve followed my forecasts over the years, you may have noticed someone missing lately—Tomoe the Weather Kitty. I’m heartbroken to share that Tomoe passed away yesterday after a long illness.
He’s been a part of so many videos and live streams, quietly sitting on the desk or curling up beside the radar. I know many of you have asked about him, and I appreciate the kind words.
“He was the best little weather assistant I could have asked for.”
Rest easy, Tomoe. You’ll be missed.