Ano nga ba talaga? Why forecasts for Ruby differ

All eyes are on Typhoon Ruby (Hagupit) as it approaches the Philippines, and the foremost question on everyone's minds is: Exactly where is it going?

 

Early Thursday, after crossing into the Philippine Area of Responsibility (PAR), I M Ready issued the following compilation of Ruby's forecast movement as predicted by various international weather agencies:

 

 

But which of these tracks is the real one, or at least the most likely?

 

The best that anyone can do is to provide an educated guess of the future based on current and previous information. Each agency relies on different measuring systems and formulas to make a careful assumption of how strong a typhoon will be and where it is likely to go. 

 

For example, aside from computer models, PAGASA also considers the tracks of previous typhoons during this time of the year.

Differences

 

"The accuracy of hurricane forecast models can vary significantly from storm to storm. For some storms, the factors affecting the hurricane track are relatively straightforward, and the models are not only accurate but they produce similar forecasts, while for other storms, the factors affecting the hurricane track and more complex, and different models produce very different forecasts," explains HurricaneScience.org.

 

"Forecast models vary tremendously in structure and complexity. They can be simple enough to run in a few seconds on an ordinary computer, or complex enough to require a number of hours on a supercomputer," adds the US' National Oceanic and Atmospheric Administration (NOAA).

 

"scientists are aware that there may be a predictability limit, at which point no amount of model improvements will improve the forecast... (But) even with a 'perfect' model that captures all of the relevant physics, uncertainties in the initial condition limit the model’s ability to create a perfect forecast, especially at longer lead times (of over five days)," HurricanScience.org concluded.

 

Also, all forecasts change as new information comes in, because fresh data gives a better idea of what's actually happening versus what was expected to happen. This, in turn, allows for forecasts to be adjusted and changed.

Similarities
 

In any case, so-called "spaghetti models" like the one above—so-named because of their resemblance to pasta—help us see the similarities and differences between the various forecast tracks.

 

As can be seen, despite their differences, the various forecast models are in agreement that Ruby is most likely to cross Central Visayas. 

 

Even the most "optimistic" forecast, from JTWC, shows that Ruby might move northward toward Bicol and eastern Luzon.

 

Also, regardless of where Ruby will make landfall, it should be remembered that Ruby has a diameter of some 600 km so it will definitely affect areas far from the point of landfall. — With CP David, GMA News