Why do weather groups give different storm track forecasts?
MANILA, Philippines – As Typhoon Ruby (international name: Hagupit) made its way into the Philippine Area of Responsibility (PAR), various weather agencies started releasing their own forecast of the storm’s path.
Dr Gerry Bagtasa of the Institute of Environmental Science & Meteorology, UP Diliman (Weather Manila and Project NOAH) tells Rappler that as early as December 1, Western-based forecast models like the Global Forecast System of the National Oceanic Atmospheric Association, the Navy Global Environmental Model of the US Navy, and the Global Environmental Multiscale Model of Canada showed that Typhoon Ruby’s path would curve to the north and miss the country.
Bagtasa adds, however, that agencies like the Global Spectral Model of Japan, the European Center for Medium Range Weather Forecasting, and our own Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA), predicted that the typhoon will make a landfall in the Visayas.
In PAGASA's forecast dated 11 am Saturday, December 6, Ruby will not pass through Metro Manila but Masbate and Oriental Mindoro instead.
Different models, data archives
But why do predictions on the storm paths vary?
The forecast models used by the different weather agencies would really vary, according to Bagtasa.
“The idea with models is that if we know the current state of the atmosphere, we can use that information to predict the future state. It’s just like when we know the exact speed of the car we are driving, we can predict our arrival time. But the thing is, we can't know exactly what’s happening everywhere in the sky. We just have weather measurements here and there, and that lack of knowledge is one big source of model error and differences,” he says.
He adds that forecasters look at previous typhoons that have similar traits and conditions with the one they’re monitoring. It is here where they "intelligently” predict the storm track.
“In any weather agency today, the job of forecasters is not to look only at their own computer model, it's more like looking at all the models that are available, then choosing which they think best suits the situation.” – Dr Gerry Bagtasa
Padua explains the same thing. He says they get the mean average of various forecast models and create their own forecast plots. Then, they weigh which model is closer to the one they’re monitoring.
“In any weather agency today, the job of forecasters is not to look only at their own computer model, it's more like looking at all the models that are available, then choosing which they think best suits the situation,” Bagtasa says.
Impact on Metro Manila
What this essentially means is that although Metro Manila does not appear to be in the path predicted by many of the Asian weather groups, it still makes sense for the metropolis to prepare.
“If the storm passes through Mindoro on the way to Metro Manila, it is likely that impact will be more winds than rain because of the effect of Sierra Madre. But if it passes through Cavite, Metro Manila [as what happened during typhoon Glenda] could be directly within the eyewall of storm; impact will be strong winds and rainfall,” Michael Padua of Weather Philippine Foundation (WPF) says.
It is also critical to note, according to Padua, that the track is only the eye. "What is more important is the effect of the storm." The effect of the storm is determined, among other things by its diameter and the size of the area surrounding the eye, Padua said.
He adds that the predicted path of the storm tends to change for various reasons. "It is important to always get the latest update." Updates are typically released every 6 hours.
According to Bagtasa, the latest storm track of JTWC is a little higher in latitude than PAGASA’s. It will pass through Batangas, then Bataan, as another US model also shows.
Metro Manila will be very rainy in both tracks since the diameter of Ruby is so huge at more than 600 kilometers, Bagtasa states. Even it it hits Mindoro, Ruby will still have a significant effect on the metropolis.
“One single afternoon thunderstorm can cripple Edsa; what more a typhoon with such enormity?” says Bagtasa.
PAGASA still official
Though reports of all weather agencies are considered valid, PAGASA remains the official government agency for weather forecasting. The public storm warning signals it releases are used by national and local government units as basis for mobilization during weather disturbances. Its forecasts are also used by Project NOAH in simulations which become basis for the storm surge advisories.
Belying a recent Philippine Daily Inquirer report which said that PAGASA is ill-equipped for storm forecasts, Project NOAH Executive Director Mahar Lagmay defended the state weather bureau saying much have happened since the time the Commission on Audit (COA) released the report that the Inquirer based its story on. That COA report is dated December 2013.
"If we are ill-equipped, my question is, how were we able to predict two days in advance those hit by the storm surge during Yolanda?" Lagmay said. – Rappler.com