Saturday, December 17, 2011

typhoon warnings: switching from wind to rain

I was awed with the immense damage Typhoon Sendong brought to the northern cities of Mindanao the past few days. Immediately, everybody point their fingers to anybody they can put their blame on. Others say PAGASA did not warn us enough. Others say the local government did not prepared enough for the impending disaster. Others point to victims who did not heed the authorities' advice. Others cast their blame on illegal (how about the legal?) loggers who wantonly denude our forests.

Is PAGASA really at fault? In fairness to them, I have seen several press releases with a warning of an impending typhoon. They even quick to raise typhoon signals even before the typhoon's landfall. So are we going to blame the local authorities? I am not so sure either. Preparedness and response are usually adapted based on experience. Unfortunately, experience about typhoon is rare in Mindanao.

But let us turn our discussion to those who are in Luzon and Visayas. Is storm signal no. 1 or 2 enough to cause massive pre-emptive evacuation? We need to accept the fact that in the Philippines, we are only concerned when a Typhoon reached a signal 3 or 4 or 5. I still remember when Typhoon Juan was approaching Luzon last year. People are glued to their radio and TV sets trying to know where the typhoon was. But let us admit that when it is on signal level 1 or 2, we let our guards down. In fact, it is only upon Signal 3 that classes at collegiate level are suspended or offices are closed. Meaning, we really don't see a tragedy coming at Typhoon Signal 2. We don't have the sense of urgency when a typhoon is tagged at the lower level. As I have said, we remained complacent and we let our guards down. Much more to areas which have not experienced severe typhoons. 

And this is where disaster strikes! Remember Ondoy? It was just Typhoon signal no. 2. This time Typhoon Sendong. Who would ever think that a typhoon with signal no. 2 can cause widespread havoc to an entire region with casualties and flashfloods unimaginable even in a typhoon signal no. 3?

Typhoon signals nowadays are so vague and misleading. The results are different. Few typhoons skirted Northern Mindanao in the past but did not cause damage at a magnitude similar to Sendong. There are typhoon signal 2 that can cause flooding while others do not. It gives us an impression that there are different types of typhoon signal no. 2. Nowadays, it is difficult to decipher when a particular typhoon signal can be destructive or not. And so you will hear several comments that "I thought it was just Signal no. 2?" 

What policy-makers failed to see is that our warning messages are not enough to warn. Can you warn somebody about flooding with signals derived from winds? Warning should be about the amount of rain. Ondoy was just storm signal no. 2 but it poured rain even greater than the usual typhoon signal no. 4. After Ondoy, Doppler radars were installed to predict rainfall volume even if the typhoon is still on the ocean. So we now have the capacity to project possible rainfall volume even before its landfall. But even knowing the main cause and installing the latest gadget, we still stick to our old ways of giving warnings. We still give warnings based on the strength of winds.  We need to redefine our warning classification that is based on the real culprit: RAINFALL INTENSITY. After all, flooding and landslide is not caused by winds.

I proposed a warning signal classification based on RAINFALL INTENSITY.  For example if a typhoon is coming, PAGASA can issue a typhoon signal and at the same time issue a rainfall signal. In this way a typhoon can have signal of 2 but with rain signal of 4 (super heavy rains). This means that we expect a moderate or strong winds but very heavy rains. So communities need to prepare even with low storm signal. I understand that PAGASA also included in their bulletin the expected rainfall volume but it has been de-emphasize and overshadowed by the strength of the winds. We need to emphasize rainfall intensity similar to the treatment we gave to wind strength. By having a separate rain warning, PAGASA can issue Rain Signal Levels even for mere LPAs.  Mind you, we have flooding episodes even without typhoons.

In Mindanao where typhoons seldom passed, rainfall intensity signal is more meaningful and definite than a storm signal (actually a storm, in hydrology, is about a rainfall event so associating it wind strength already deviate from its meaning). If a rainfall signal is raised, local authorities based on their knowledge of topography and hydrology can decide among themselves if a particular rainfall intensity merits pre-emptive evacuation. I do not see the logic of preparing for flood with information based on winds. When a community is accustomed to rainfall signals, they themselves can decide to evacuate or not. They do not need to compute based on mm/hr which is hidden somewhere inside weather bulletins issued by PAGASA and where only hydrometeorologist could understand. It become their instinct.

Yes, PAGASA and NDRRMC forewarned an approaching typhoon, but did they emphasize that the rains that are coming are at a magnitude that cause floods similar to Ondoy? Mahangin ba sa labas? Kung hindi, bakit kaya bumaha?

Please read: topography of flashfloods. and  using rainfall to save yourself from flooding


  1. Bayaw, ang mga bagyong Violeta at Winnie noong Nov 2004 na tumama sa Quezon province ay nasa Signal number 2 yata. Nagkaroon ng mga landslide at maraming namatay, nasa higit 1000. Sa pagkakatanda ko, hindi naman ganoon karami noon ang bumubuhos na ulan kada oras (low rainfall intensity) pero matagal ang ulan (high rainfall duration), palibhasa mga ilang araw lang ang pagitan ng dalawang bagyo. Dahil low rainfall intensity, tumaas ang infiltration rate ng lupa. Samakatuwid, sinipsip ng lupa ang tubig ulan hanggang sa ma-saturate at hindi na maka absorb pa ang lupa kaya lumambot ito. Theoretically (i.e. kung maayos ang vegetation), iyung hindi na ma-absorb ng lupa e dadaloy na lang bilang surface run-off at maghahanap ng daan para makarating sa mga creek at ilog. Sa mga lugar na naperwisyo na ang vegetation, wala ng mga ugat kaya hindi na napigilan ang pagbigay ng lupa.Kaya bayaw,sang ayon ako sa suggestion na gamitin ang impormasyon sa rainfall intensity para sa paghahanda ng komunidad tuwing may parating na bagyo. Totoong walang dalawang bagyong magkapareho pero kung may mga historical data na maaring pagbasehan, makakatulong din kung posible na isama ang estimated rainfall duration sa mga impormasyon na isasama sa paghahanda kapag may bagyo.

  2. salamat sa info bayaw. siguro idagdag natin ang rainfall duration sa criteria lalo na para sa landslide forecasting. Pero definitely ang hangin as criteria medyo alangan ako. Imagine, another example of typhoon signal 2 with problematic consequences? Dapat talaga maibase na sa ulan hindi lang sa hangin.

  3. ang galing bayaw..salamat.. dami ko natutunan..

  4. Thank you!It has been awhile since I have read an article/analysis as informative as the one you have written, even the 1st grader would understand and identify the importance of rainfall in relation to floods.

  5. I remember back in 2007 when we experienced flood, I wrote to our Mayor informing him that it is not the typhoon per se that brings flood but it's the rain espec. that we're in an area were typhoon does not cross our path w/ the exception of Sendong.