PAGASA's imperfect storm of typhoon forecasts

Just how well does the Philippines' weather bureau see into the stormy future?

In its Tuesday severe weather bulletin, PAGASA predicted that Typhoon Chedeng (international name: Songda) will make landfall between the regions of Isabela and Aurora by Friday afternoon.

Different weather monitoring agencies worldwide, however, report that the storm will merely graze Luzon's eastern coast—and thus will not make landfall at all—before heading to Taiwan and back out to the northern Pacific Ocean.

Then on Wednesday morning, PAGASA presented an alternate scenario,with the storm passing through Metro Manila. Then it revised its forecast in its evening bulletin, saying the storm is proceeding along its west-northwestern track, away from the nation's capital.

Conflicting and changing predictions confused anxious Luzon residents and raised doubts about the credibility of PAGASA, whose predictions can shut down local economies and send schoolchildren home.

"PAGASA is such an unreliable weather agency. Storm tracks by more competent weather agencies clearly predict it will NOT make landfall," @kevinyapjoco pointed out on Twitter.

Contradictory forecasts

At least three different international weather bureaus issued reports that contradicted that of PAGASA.

The Japan Meteorological Agency (JMC), the Joint Typhoon Warning Center (JTWC) and the Central Weather Bureau of Taiwan all predicted that Typhoon Chedeng will not make landfall in the Philippines, based on a 70 percent probability track area they released separately.

In an interview with 24 Oras, however, PAGASA officer-in-charge Graciano Yumul Jr. said these weather agencies use mathematical models to predict the storm's track, while PAGASA uses a combination of mathematical models and instrumentation to identify where the typhoon will go.

"Iba po ang metodolohiya ng ating pag-compute. Gusto nating ipaalala na 'yung track line po na nilalagay [nila], 'yan po ang mata ng bagyo. Ang atin pong bagyo ay 600 kilometers ang diameter. Sabihin na po natin na hindi tatama ang bagyo, pero 'pag sinundan nyo po ang ginawa nilang trackline, 'yun atin pong eastern seaboard ay basang-basa pa rin," he explained.

In making the prediction, JMC used a "Typhoon Ensemble Prediction System," which uses multiple numbers of "initial perturbations" to determine a weather disturbance's future actions.

Initial perturbations are atmospheric samples of a certain weather disturbance that is recorded and extrapolated using equations of fluid dynamics and thermodynamics in order to predict its future state.

Japan's ensemble system uses ten perturbed members and one non-perturbed member (control data), which can reduce errors in positioning the disturbance by an average of 40 kilometers five days into the future.

In contrast, PAGASA is using a host of prediction models, satellite imagery and radar technologies in determining specific characteristics of a typhoon, according to Rene Paciente, PAGASA forecaster, in a phone interview with GMA News Online.

While the agency also uses a number of numerical weather prediction models—the latest of which is the Weather Research and Forecasting (WRF) model acquired in September 2010—it combines other data from satellites and doppler radars as well to make a final prediction.

"We also have weather maps, where we look at the upper levels of the atmosphere. Sometimes [the conditions in these levels] dictate the direction of the typhoon," Paciente said.

He said the agency also uses doppler radars—positioned in Subic, Baguio, and Aurora—to measure the amount of rainfall, the location of the center of the typhoon, and its windspeed.

Paciente stressed, however, that Typhoon Chedeng is currently too far off the range of the radar in Baler, Aurora, which requires a distance of 400 kilometers for atmospheric data to be picked up.

Hourly forecasts

Yumul said that in October 2010, only PAGASA correctly predicted that Typhoon Juan will make landfall in Isabela.

He said the advantage of PAGASA's predictions is that they are monitored every hour, on the hour, compared with other monitoring agencies that monitor only every six hours.

Despite this, Paciente said that one of the main limitations of their prediction models is that its outputs are often inconsistent.

"Every output nagbabago, depende yan sa parameterization. Kung minsan ganito ang track, sa susunod na output iba naman. Ganun talaga ang mga models," he said.

He highlighted their earlier prediction that Typhoon Chedeng will hit Metro Manila when the early morning output of the models depicted the typhoon moving westward.

"Right now, it is moving in a west-northwest direction, so we changed our advisory," he said.

Judgment calls

But PAGASA's predictions aren't borne out of mathematical models and instrumentation alone.

Paciente claimed that the other international agencies offered a different forecast for the typhoon because the storm has a slim chance of affecting them.

"Tayo, conservative tayo. 'Wag mong basta-basta palilihisin yung bagyo, kasi bansa natin ito. Nagiging alerto kami, kasi posible ring tumuloy din sya westward, pero nung kumilos sya pa-northwest, binago na namin," he explained.

"Inaasahan pa rin namin na tatama ito (Typhoon Chedeng). Judgment call ng forecaster iyon, kasi mahirap naman, kailangan isa-alang-alang mo rin 'yung kapakanan ng mga tao. Habang malayo pa 'yung bagyo tapos pinalihis mo, eh paano kung tumama? Mas maganda patamain mo na, so kung tumama man, nakahanda na sila," he revealed.

"Kami po sa PAGASA ay matutuwa kapag hindi tumama sa kalupaan ang bagyo, subalit ang amin pong track lines at ang aming mga synoptic stations ay talaga pong nagpapakita na tatama ito sa Isabela at Aurora," Yumul said.

In adopting a cautious stance on forecasting weather disturbances, PAGASA is perhaps trying to learn from the dire lessons of Tropical Storm Ondoy in 2009, and the other missed predictions over the years.

Last year, President Benigno Aquino made an example out of former PAGASA head Prisco Nilo, who incorrectly predicted that Typhoon Basyang would not hit Metro Manila, before Aquino relieved him of his post.

Given its meager budget and the skyrocketing costs of acquiring equipment, it may take time before the country's weather bureau can issue forecasts with more hits than misses. - HS, GMA News