Onset of the Rainy Season

Rains loom on the horizon making this year one of the shortest hot, dry season and among the warmest year so far. Last April 27, our Automated Weather Station (AWS) in Lucena City, Quezon (LCP) has recorded one of the hottest air temperature (TL) reading at 43.2°C. The highest heat index (HI) occurred in San Isidro, Nueva Ecija (NEECO) with 56.8°C on May 19, followed by Pilar, Capiz (LFUG) on May 09, with 50.8°C, and DOST-ASTI in Quezon City for NCR with 47.2°C respectively. The numbers are quite staggering, but as we build on the data we will be able to understand further the dynamics of our ever-changing climate and weather in the Philippines.

While very warm and humid conditions are still being felt lately, we can hope for a respite in the coming weeks as we welcome the start of the rainy days. To help us understand more about the upcoming rainy season, here are some of the crucial factors affecting the local weather in the Philippines during the months of May to June. What can we expect in the following days ahead? Let’s all find it out.


Trough associated with Low Pressure Areas (LPAs)

The transition period has already been reached for almost a month now, as indicated by the prevalence of several Low-pressure systems (LPAs) across Asia. Most of it have extended over the Northern-most areas of the Philippines in the last couple of weeks. This has brought wet weather in the region particularly along Batanes-Calayan and Babuyan Island Group. Back in late May, some landslips and rockslides have already been occurring in the high mountains of Benguet as rains continue to fall saturating the ground.

The steady increase of thunderstorm activity is having such an impact in the Southern regions of China. Development of Frontal Lows becomes more frequent while the Stationary (Mei-Yu) Front has been quite active lately. All these are expected to bring about the threat of flooding in the region in the coming days.

It’s the month of June now, the Rainy Season Front have now migrated further North of Taiwan as East Asia welcomes its Wet Season well into the month of July. And notably, it stays warmer for the most part of Southeast Asia in anticipation of the Southwest Monsoon, we call ‘’Habagat,’’ in the Philippines.


Fig. 1) Surface Chart Analysis as of 08:00 A.M. PhT, 08 June 2017, indicating typical weather pattern across Southeast Asia this time of year. Areas in the Philippines in particular along the Western sections of Luzon and Visayas, classified as under Type I Climate should see an increase in thunderstorm activity, thus the constant afternoon, late evening or early morning rains.


Shift of surface winds to a southwesterly orientation

 Based on our weather observation in the past four weeks, the shift in the wind direction coming from the Pacific Ocean has now been replaced by a Southerly orientation prevailing mostly across the Sulu Archipelago, while the Southwesterly surface wind continually develop over the Gulf of Thailand with its leading edge occasionally reaching the South China Sea and the West Philippine Sea. This is a clear indication that the hot, dry episode is coming to a close. However, the onset of  Habagat will depend on the development of Tropical Lows to the East of the Philippines, as the axis of the Intertropical Convergence Zone (ITCZ) has already reached Western Micronesia several times over.

It is also important to note that these moisture-laden winds are being pushed by strong High Pressure Areas (HPAs) over the Indian Ocean. As they travel vast oceans they gather warm moist air, thus the build up of rain-bearing clouds. They can produce ‘’on-and-off” heavy rains and gusty conditions along the immediate coast especially over the Western seaboards of Luzon and Visayas areas classified as under Type I Climate. Right now, the advance of Habagat is contained over the Andaman Islands and the Bay of Bengal, and has no direct effect to the Philippines.


Fig. 2) Variable winds blowing from the South and Southwesterly direction along the Western side of Luzon and Visayas becomes dominant in anticipation of the end of hot, dry days. Over the Sulu Sea, a developing Trough extends well into a Weak Low Pressure Area (LPA) located East of Mindanao.


Electrical storm

According to Byers, H. R. and R. R. Braham Jr. (1949), a thunderstorm can also be referred to as an electrical storm, where a cumulonimbus cloud is being accompanied by lightning and thunder, usually with strong gusts of wind, heavy rain, and sometimes with hail. With this, brace for more thunderstorms developing in the coming days across the archipelago due to increased atmospheric instability. Keep in mind that a thunderstorm can develop and dissipate quickly, adding to a certain degree of difficulty in forecasting where it may have a significant impact.


Fig. 3) Lightning strike density in several locations across the Philippines. This usually occurs during midday through late afternoon to early evening. The concentration of strikes sits along a developing Trough extension from a forming Weak LPA off Eastern Mindanao. The graphical representation do not necessarily denote actual impact of strikes on the ground/surface.


Keeping an eye on the number

According to our weather bureau, DOST-PAGASA,  there are requisites that should be met to declare the start of the rainy season. For one, the eight (8) select locations across Luzon and Visayas (facing the West) should record at least 25.0 mm reading in the last 24 hours for five (5) consecutive days. With this, beginning 04th of June, 2017, we have been monitoring closely the rainfall totals in eight (8) select automated weather station (AWS) located along the Western section of Luzon and Visayas, as shown below:


  • Closest AWS to areas identified by DOST-PAGASA

Fig. 4) Areas of interest. Monitoring the amount of precipitation in the past 24-hours ending at 08:00 A.M. PhT daily.

As for now, the figures indicated above do not show that much rainfall accumulation, but rest assured that in the coming days well into the first week of July, the rainfall totals should reach its tipping point as the Monsoon rain get its act together.


Climate Type

The Philippines is an archipelago of over 7,641 islands as of recent statistics, and is located between four to twenty-one degrees North in the Western Pacific, comprising of two large islands, such as Luzon and Mindanao where mountain ranges rise from 1,800 to 3,000 m above sea level. The Visayas is also composed of hundreds of smaller islands with indented coastlines. On a normal year, the Philippines receives over 1,000 mm of rainfall, with the Southern islands having more equatorial climate, thus having more rainfall all year round.

Every year, torrential rains come from Tropical Cyclones (Typhoons) on this part of the Pacific, where high winds and flooding conditions impact a huge population in the country. These Typhoons develop over the warm Pacific Ocean and move Westward across the Philippines well into the South China Sea. Through time, many parts of the country have seen devastation, and are no stranger to tragedy brought about by these vast weather disturbances which cause massive loss of life and property.

According to Francisco A. Moog of the Bureau of Animal Industry’s Research Division in 1999, the tropical climate in the Philippines are dominated by rainy and dry seasons with mean annual average temperature of about 27°C. The hottest months are April, May and June with average temperatures ranging from 27.8 to 28.4°C, where the coldest months are December, January and February with average temperature ranging from 26.1°C to as low as 25.5°C.

Based on the study, the Philippines is divided into four climatic types, depending on how rainfall is distributed throughout the year, see (Fig.5).

  • Type 1 – Two pronounced seasons, wet and dry, with maximum rain period from June to September and a dry season which lasts from 3 to 6 months;
  • Type II – No dry season, with a very pronounced maximum rain period that occurs in December and January;
  • Type III – Not very pronounced maximum rain period, with a short dry season lasting from 1 to 3 months;
  • Type IV – Rainfall more or less distributed throughout the year.


Fig. 5) Philippine map showing four (4) distinct Climate Types from the document written by Francisco A. Moog in 1999 for FAO.org, where its being sub-divided into three (3) zones, namely: Wet, Moist and Dry. This data is crucial particularly in Agro-Ecological zones, where food production and other major industry as main drivers of economic growth.



According to a study, almost 60% of the world’s population live in monsoonal climate, and this includes the Philippines. Monsoons, according to United Kingdom’s Met Office, comes from the Arabic word ”Mausim” which translates as ‘‘Season,” which is suggestive of the seasonal nature of the monsoon and its associated rains.

Monsoons lead to distinct wet and dry seasons in many areas throughout the tropics and are most often associated with the Indian Ocean. It can be understood right away that these wind system correlates with the movement of migrating rain-bearing clouds from the Indian Ocean, while Southeast Asia in general receives a secondary Southwest Monsoon every year, which includes again, the Philippines.

Just before its arrival, the warm and moist monsoon heat builds up and throughout its progress, as Tropical activity becomes more prevalent in the Pacific Ocean, a heavy surge of moisture-laden nimbus clouds take hold over vast ocean waters as it reaches land mass, it should bring about heavy burst of rains and sometimes ”on-and-off” gusty conditions and torrential downpours. That’s the time where flooding and landslides along steeper elevations become an issue, especially in areas where vulnerable population live. It is also the time where the seas around becomes unsettled and rough.

As for small and medium sea crafts, this becomes a difficult challenge, in particular with fisherfolk. Traveling from one place to another is a life-threatening gamble as the swells and waves become treacherous for sea travel. Safety should always come first in a country surrounded by waters, and it is the sea where most of our people’s livelihood come from.

Fig. 6) Moderate to heavy precipitation has been observed because of increased thunderstorm activities in the country for the last 24 hours (June 07-08, 2017).

The western part of Luzon, Palawan, and the Visayas islands have Type 1 climate, with marked dry and wet seasons. Rainfall in these areas occurs mostly during the Southwest Monsoon “Habagat” season. With Type 1, there are two (2) pronounced seasons, wet and dry, with maximum rain period from June to September and a dry season which lasts from 3 to 6 months.

During November to April, areas under Type 1 climate (Luzon and Visayas) have dry conditions and wet conditions for the rest of the year.


General Outlook moving forward

And so, what can we expect in the coming days ahead? There will be frequent days of heavy thunderstorm activity due to strong convection and thermal heating of land mass. We can also experience thundery showers accompanied by sudden gust of wind especially along the sea lines, with wide-reaching swells build up across exposed sea regions of the archipelago making sea travel a lot difficult. Rainfall is slightly below normal on the first part of the wet season. As the Southwest Monsoon gradually sets in later this month through early part of July, the nights can be relatively warm, in particular with dryer days. There will be periods of having plenty of sunshine in between rains. This happens during a “Monsoon’’ break, where after a succession of moderate to heavy rains, normal weather condition can be observed, with weather becoming fresher overall.



Fig. 7) Rainy days are here. Finally, after several months of scorching heat, temperatures should gradually reach its normal threshold. (Photo inset from www.shutterstock.com)

Rainstorms can be frequent while ambient temperature start to improve and stabilize especially during the later part of July through August, when the surge of Habagat is at its peak.  It will also be the time of year when Tropical Cyclones become active. Given the unpredictable nature of the upper atmosphere, there will always be a break in the weather-and that’s guaranteed. For now, we wait for the great migration of heavy rain-producing monsoonal system.

As the rains pushes forward, there are worthy initiatives that can be done to help build a #WeatherWiser nation. It pays to be informed and to stay ahead of the weather.





Byer, H. R., and R. R. Braham Jr. 1949. The Thunderstorm. U.S. Government Printing Office, 287 pp.





Created by the WPF Meteorology Team

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