Tropical Storm Washi – why was it so bad?

Composite radar and satellite image of TS Washi [NASA]

The death toll from Tropical Storm Washi which hit the Philippine island of Mindanao on Thursday at around 1500 UTC now exceeds 500.

Reports suggest that flash floods have washed away entire villages and many more people are reported missing, feared drowned.

River levels rose dramatically after 12 hours of torrential rain, causing rivers to burst their banks.

The northern cities of Iligan and Cagayan de Oro appear to have been particularly badly hit. The vast majority of the dead were from these two cities. It appears that many victims of the flash floods were asleep in their beds when the waters swept through.

The question arises: Was Washi a particularly intense storm or were there other factors at work?

Usually with tropical cyclones, the wind plays a contributing factor. This does not seem to have been the case with Washi. Winds were very close to those predicted, reaching around 110 kph – strong but not likely to cause significant damage. This is why Washi never reached ‘typhoon’ status; its winds were not strong enough.

The rainfall reports from the Philippines appear to have been unremarkable for such a system – 24 hour totals of up to 200 millimetres were reported in a few locations in northern Mindanao but these values are not untypical of tropical storms.

Nevertheless, it is possible that a localized region of much more intense rainfall hit this area. Evidence for this is supported by NASA’s Tropical Rainfall Measuring Mission (TRMM) satellite whose images were used to create this three-dimensional image:

3D image of TS Washi [NASA]

It shows the vertical extent of thunderstorms over the region, extending to heights of over 15 kilometres, giving the potential for rainfall of 50 millimetres per hour.

Even with exceptionally heavy rainfall, the death toll seems surprisingly high and this may be the result of other factors.

Firstly, and not to be underestimated, the floodwaters struck at night. This is sure to have caused panic and confusion.

Secondly, there have been reports of complacency. Most of the tropical storms which strike the Philippines – and there are around 20 each year – hit the northern islands.  Typhoons Nesat and Nalgae, which hit Luzon in the space of a week, were also responsible for torrential rain plus winds close to 200 kph.

Perhaps it could be argued that inhabitants of Luzon are more used to dealing with the effects of such storm systems than their southern neighbours.

Perhaps the biggest factor in this dreadful event was that the island of Mindanao is highly susceptible to the risk of flash flooding and landslides.

In an Office for Civil Defence report in February 2010, at least 291 barangays (villages) in the northeast of the island were identified as ‘flood prone’ whilst another 183 were reported as having a ‘potential to landslide occurrence.

It may have been a dreadful coming together of factors which resulted in Tropical Storm Washi becoming such a tragic event in the country’s history of battling against severe weather.