Thunderstorms, unfortunately, are difficult to predict. And in a country like the Philippines with inadequate infrastructure, that's even more bad news.
Thunderstorms are common in the country, and so are floods. But the severe thunderstorm that struck northern portions of Metro Manila on Saturday, August 30, still left many stunned as it unleashed torrential rain that caused flash floods in Quezon City.
The Philippine Atmospheric, Geophysical, and Astronomical Services Administration (PAGASA) called Saturday's rainfall "unprecedented."
In just three hours, between 1 pm and 4 pm on Saturday, PAGASA's Science Garden Station in Quezon City recorded 134.2 millimeters (mm) of rainfall.
That is equivalent to nearly one week's worth of rain, "based on the monthly normal of 568.5 mm" for August, the weather bureau said on Monday, September 1.
Much of the rain, or 96.6 mm, fell from 2 pm to 3 pm. This exceeded the peak hourly rainfall of 92 mm at the Science Garden Station during the devastating Tropical Storm Ondoy (Ketsana) in September 2009.
Some streets turned into rivers, traffic was paralyzed, vehicles became submerged, and commuters were stranded or forced to wade through floodwaters. The scenes looked like a tropical cyclone had headed Metro Manila's way, but there wasn't one -- it was a severe thunderstorm.
Thunderstorms, unfortunately, are difficult to predict. "In fact, it's almost impossible," said Gerry Bagtasa, a professor at the Institute of Environmental Science and Meteorology of the University of the Philippines Diliman.
Picture this analogy provided by Bagtasa: You heat a pot of water on your stove. Eventually, the water will start boiling and bubbles will form.
"Think of the atmosphere as parang water, tapos 'yung thunderstorm, siya 'yung parang bula na lumalabas.... 'Pag 'yung nagpapakulo tayo ng tubig, hindi mo mahuhulaan saan lalabas 'yung bula eh," he told Rappler on Tuesday, September 2.
(Think of the atmosphere as water, and then thunderstorms as bubbles that emerge. Whenever you boil water, you won't be able to guess where the bubbles will form.)
Bagtasa also pointed out that thunderstorms are small and localized systems, "just a few kilometers" in size and affecting only certain areas. Forecasting is easier when it comes to much larger weather systems like low pressure areas and tropical cyclones -- something that is not unique to the Philippines or PAGASA.
"Actually, kahit na thunderstorm say, sa Japan or sa US, hirap din silang i-forecast...'yung talagang uulanin 'yung isang lugar lang, tapos that's it (Actually, even countries like Japan or the US have difficulty forecasting thunderstorms, when really just one area will experience heavy rainfall, and that's it)," Bagtasa said.
"Small system, but it carries a lot of water and it will dump the water in a relatively short amount of time," he added.
Last Saturday, the first PAGASA thunderstorm advisory that included Metro Manila came shortly before noontime, warning that moderate to heavy rain was expected to begin in two hours. Another advisory was released around an hour later, saying that heavy to intense rain was looming. Afterwards, an advisory at 2:10 pm showed that intense to torrential rain was already affecting portions of Metro Manila and nearby provinces.
PAGASA has emphasized the importance of monitoring thunderstorm advisories and rainfall warnings. This is especially crucial, given the shorter lead time when it comes to preparing for thunderstorms.
It also pays to know which areas are prone to hazards such as floods and landslides, and to be on alert during periods of heavy rain, even in places usually considered "safe."
PAGASA noted that Saturday's severe thunderstorm "caused significant flooding, particularly in areas near creeks of the San Juan River system, which are already classified as highly susceptible to flood based on GeoRiskPH's flood hazard map." Flood monitoring stations in the Tullahan and Pasig rivers "reached alarm and critical levels."
Among the flooded areas were Araneta Avenue, Mother Ignacia Avenue, EDSA-Centris, and Elliptical Road -- long notorious for being inundated during heavy rain. But several other areas which usually see minimal to no flooding, like Katipunan Avenue, were considerably affected too.
In total, floods affected 36 of Quezon City's 142 barangays, equivalent to 25%.
"Hindi kinaya ng drainage system ng lungsod ang napakaraming tubig-ulan sa napakaikling oras kaya nagresulta ito ng malalang pagbaha, maging sa ilang lugar na hindi karaniwang binabaha," the Quezon City government said on Saturday evening.
(The city's drainage system couldn't handle the significant volume of rainfall that fell in such a short time, which resulted in massive flooding, even in some areas that don't usually get flooded.)
Quezon City assured residents that it is pursuing a Drainage Master Plan. The local government unit described it as a long-term and effective solution to flooding, in contrast to flood control projects that Mayor Joy Belmonte had said weren't approved by her LGU and could be anomalous.
Infrastructure is a critical area where the Philippines differs from developed countries like Japan, a painful reality that became even more evident in recent weeks owing to the flood control controversy.
What occurred in Quezon City could happen again. Rainfall of such magnitude -- 96.6 mm in one hour -- "corresponds to a return period of 10 to 15 years," according to PAGASA.
The World Meteorological Organization (WMO) defines a return period as "the projected period of recurrence of a particular event."
But the WMO also notes, "Great care needs to be taken in interpretation of this statistical result; in particular, a return period of, say, 50 years does not mean that the next event will occur in 50 years, it could occur next year, albeit with a low probability."
In other words, the return period represents the chance of a similar event happening in any given year. If it is a 10-year event like Saturday's rainfall, there is presently a 1 in 10 -- or 10% -- chance of it happening in any given year. It does not mean that it will happen only every 10 years.
That's statistics, but Bagtasa also said "it's difficult to say how often" such an event will occur, especially with a changing climate.
"Ang daming unprecedented na measurements, not only here in the Philippines, pero sa ibang bansa rin -- mga unprecedented measurements ng extreme rainfall, extreme temperature.... Because of the changing environment na tinitirhan natin, parang nagbabago rin 'yung statistics, in the process of changing din siya," he explained.
(There are many unprecedented measurements, not only here in the Philippines, but even in other countries -- unprecedented measurements of extreme rainfall, extreme temperature. Because of the changing environment we live in, statistics are also in the process of changing.)
By the time a severe thunderstorm of that magnitude strikes again, will Filipinos be prepared? - Rappler.com