Forecast: Ketsana moves into Asia

At least 54 people have been killed in the storm that destroyed property and homes [AFP]

A state of calamity has been declared in the Philippines following the passage of tropical storm Ketsana, which ripped through Luzon on Saturday causing the worst flooding in living memory. As the storm hit, it was packing winds of around 100kph.

However, the winds were not really the issue on this occasion – it was the intensity of the rainfall that has lead to more than 50 deaths by the time of writing.

This figure is expected to rise further over the coming days.

This is a country which is used to heavy rainfall. This is seventeenth storm of the current Northwest Pacific tropical storm season. So why have they been so badly affected on this occasion? 

This has been a particularly wet rainy season. Many rivers and dams where already close to being breached before the arrival of Ketsana.

At the height of the storm, Manila saw 350mm of rain, almost a month’s worth, in only six hours. The September average is 356mm.

To make matters worse, this is an absolutely massive storm system, now engulfing much of the South China Sea, and once it started raining it was reluctant to stop.

Non-stop rain

Many areas had over 36 hours of non-stop heavy rain. One weather station close to Manila reported an incredible 455mm – almost half a metre – in only 24 hours.

At the height of the storm, Manila saw almost a month’s worth of rain in only six hours [AFP]

The eye of the storm has now moved out into the open waters of the South China Sea but that does not mean that the rain has stopped just yet. In fact, many areas can still expect to see heavy and thundery showers or longer spells rain over the next few days.

The outflow from the storm is throwing out a long swathe of shower clouds back across the Philippines as warm, moist air trails behind.

Floodwaters are now starting to subside, but Manila can still expect to see more heavy showers on Monday and although there should become fewer and further between, showers are also likely on Tuesday.

I suspect that Wednesday could well be the first predominantly dry day to come.

As the storm makes its way due west, it is expected to re-intensify and it could even become a typhoon for a short while, with winds forecast to reach 110kph with gusts around 140kph.

This would make it the equivalent of a category one hurricane on the Saffir-Simpson scale, but I suspect that it will then become a tropical storm again by the time it hits Hainan and Vietnam on Monday and Tuesday.

Rainy season

Flooding is likely in those areas, along with parts of southern and later eastern China – Hong Kong will have heavy rain on Monday – before the system eventually wrings itself out across Laos and Myanmar by next weekend.

Meanwhile, the rainy season is far from over, apart from a slight lull between January and April, and tropical storms and typhoons can be expected at almost any time of year as they readily develop in the warm waters of the northwest Pacific.

In fact, the next one is already brewing close to Guam and the Northern Mariana Islands. Thankfully it looks likely to pass to the north of the Philippines.

At this early stage, Tropical Depression 18W as it is currently known, appears to be heading towards Taiwan. It may well then curl towards Japan in about ten day’s time.

We will of course be keeping a close eye on it, but these things are very unpredictable this far out.