CSR

Signs the rainy season is here

What are the weather patterns that say the hot dry season has left the building? Here are four things marking the onset of the rainy season.

  • Frequent thunderstorm activity
    Afternoon and evening thunderstorms, some heavy, over a wide area of the country in recent weeks are a strong indication that the hot, dry months are over.

    Rainy season 1
    FIGURE 1. Areas of observed thunderstorms around the Philippines.

 

  • Trough associated with Low-Pressure Areas (LPAs) across Asia now extend over Northern Philippines
    Low-pressure systems across Asia have been prevalent and persistent for the past weeks. With its trough now extending across Northern Philippines, enhanced atmospheric instability over Luzon and Visayas, especially along the western sections, will likely make it more conducive for thunderstorms to develop.

    Rainy season 2
    FIGURE 2. Location of the trough (red broken line) and its evident southwesterly windflow or southwesterly surface winds.

 

  • Shift of surface winds to a southwesterly direction
    According to recent weather data, there is now a southwesterly surface wind direction across the South China Sea, and the West Philippine Sea — one of the important factors that signify the beginning of the rainy season. Also, much of the rainfall in Western Luzon and Western Visayas is attributed to the Southwest Monsoon (Habagat). These moisture-laden winds that originate from the South Indian Ocean will tend to sustain thunderstorm activities across the western sections of Luzon and Visayas.

    The climate of these areas is classified under Type I, which has dry conditions during November to April, and wet conditions during the rest of the year.

    FIGURE 3. Graphical representation of the southwesterly winds over the West Philippine and South China Seas, as shown on the current GFS Model. Source: earth.nullschool.net
    FIGURE 3. Graphical representation of the southwesterly winds over the West Philippine and South China Seas, as shown on the current GFS Model. Source: earth.nullschool.net
  • Presence of Intertropical Convergence Zone (ITCZ)
    The Intertropical Convergence Zone (ITCZ) has been prevailing east of Mindanao for more than two weeks. This causes frequent thunderstorm activities along the areas of El Niño-stricken Mindanao since early May.

    The presence of ITCZ across the area is one of the factors that contribute to the instability of the atmosphere across the country.

    FIGURE 4. This WPF Graphical Satellite Analysis from last May 19 shows the location of the ITCZ.
    FIGURE 4. This WPF Graphical Satellite Analysis from last May 19 shows the location of the ITCZ.

     

Overall, these observed current weather patterns prevailing over Southeast Asia indicate that the rainy season in the Philippines has finally set in. We can finally say “goodbye and good riddance” to the hot dry season and, yes, change is coming, at least in the weather.