Onset of the Rainy Season – Are You Weather-Ready?

The suffocating heat of the hot dry season will be no more as the rainy season is just around the corner. Gone will be the days of prevailing humid feeling and we will soon be greeting cool breezes and refreshing drizzles as the rainy season commences!

While the 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. According to a recent press release from PAGASA, the rainy season is expected to start this last week of May.

The indicators are already in, which perfectly demonstrate the transition period has already begun for several days now. These include prevalence of several low pressure areas (LPAs) across Asia. Most of the LPAs have also extended over the northernmost areas of the Philippines in the last couple of weeks, bringing constant heat build up along the higher elevations of the Cordillera mountain ranges. This heat build-up causes locally-driven thunderstorms, bringing rainy days on several occasions, particularly in the Benguet area. According to recent data observations, these are being influenced by late effects of the La Niña phenomenon. This means that even during the prevalence of the hot dry season, there are places along the Westerns shores of Luzon, Palawan, and North-Western edges of Mindanao that will receive near above-average rainfall.

To help us understand more the upcoming rainy season, here are some crucial factors affecting the local weather in the Philippines during the months of May to July.

Bye, La Niña

Citing the Climate Prediction Center (CPC) of the National Centers for Environmental Prediction (NCEP) of the National Oceanic Atmospheric Administration (NOAA) in Maryland, USA, in its recent report, a Final Advisory on La Niña has been reached based on its release dated May 10 2018 (See Figure 1). This means that the indicators show that near-to-below average sea surface temperatures (SSTs) along the Equator in April favored an El Niño Southern Oscillation (ENSO) Neutral condition through September-November this year. This departure from La Niña may not be immediately felt. In fact, while its effects are dying off, the Philippines can still feel its influence up to late June this year.

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Shift of surface wind direction

Based on our weather observations in the past four weeks, the shift in the wind direction coming from the Southern Hemisphere has now been replaced by a southerly orientation prevailing mostly across the Sulu archipelago. This shift is a clear indiation that the hot dry episode is coming to a close.

Being an archipelagic region known for having vagaries in weather, there could be drastic changes in the Philippines as the new season begins. With this, we should all be ready not just with our rain gear, but with the mindset that now is the time to be more wary of possible sudden changes in the weather.

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Figure 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. A developing Trough extends well into a Weak Low Pressure Area (LPA) located South of Marianas Islands.

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. Monsoon rains across the Indian Ocean is vital to millions of people who rely to life-giving waters. Without them vast tracks of agricultural land will dry up and vegetation will die off and wither due to extreme summer heat where water becomes a scarce resource.

Electrical Storm

According to Byers, H.R. and R.R. Braham Jr. (1949), a thunderstorm can also be referred to as an electrical storm (see Figure 3), 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.

Figure. 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 the far Eastern side of Mindanao. The graphical representation do not necessarily denote actual impact of strikes on the ground/surface.

Figure. 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 the far Eastern side of Mindanao. The graphical representation do not necessarily denote actual impact of strikes on the ground/surface.

Climate Type

The Philippines is an archipelago 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 3,000m about sea level. The Visayas is also composed of hundreds of smaller islands with indented coastlines. On a normal year, the Philippines receives over 1,000mm of rainfall, with the Southern islands having a 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 condition 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 the tragedy brought by these vast weather disturbances which cause massive loss of life and property.

Based on studies, the Philippines is divided into four climatic types, depending on how rainfall is distributed throughout the year (See Figure 4):

  • Type I – 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
Figure. 4 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.

Figure. 4 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.

Monsoon

Monsoons, accoridng to the United Kingdom’s Met Office, came 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 systems 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, a heavy surge of moisture-laden nimbus clouds take hold over vast ocean waters. This happens as tropical activity becomes more prevalent in the Pacific Ocean and as it reaches land mass, it should bring about heavy burst of rains.

What’s to come

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 sea lines. This is also characterized by wide-reaching swells build up across exposed sea regions of the archipelago, making sea travel 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 sunshine in between rains. This happens during a “monsoon break”. This is after a succession of moderate to heavy rains, normal weather condition can be observed, with weather becoming fresher overall.

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 the year when Tropical Cyclones become more active. Given the unpredictable nature of the upper atmosphere, there will always be a break in the weather. For now, we wait for the great migration of heavy rain-producing monsoonal system.

It’s not intuitive to ignore the signs let alone the fact that weather for the short-term and climate for the long run, both affect our everyday life. It is a force that is constantly changing in seasons. The only difference is that year after year, the weather pattern is being influenced by the changes in temperature of the Central Pacific, a major determinant which scientists from around the globe keep a tight watch. This is crucial in preparedness measures before any of these storms affect our part of the world.

As the rain 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.


By: Adonis S. Manzan

References:

Byer, H. R., and R. R. Braham Jr. 1949. The Thunderstorm. U.S. Government Printing Office, 287 pp.
http://www.fao.org/ag/agp/agpc/doc/counprof/philippines/Philipp.htm#fig5
http://www.metoffice.gov.uk/learning/learn-about-the-weather/monsoons
http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.pdf

 

 

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