Koppu and the potential for extreme rainfall

Koppu, known as Lando in the Philippines, was a super typhoon as it crashed onto Luzon’s east coast. Winds were in excess of 240 km/h. Such strong winds are commonplace in this part of the world, and Koppu would surely fade from the memory, if it was not for the extreme rainfall which has accompanied this storm.

Rain is still falling along the west coast of Luzon, but it is likely that some parts of the island have already experienced one metre of rainfall. Official recordings in the ‘City of Pines’, Baguio, have exceeded 850mm in just over three days, with plenty more to come.

To place that into context, London receives an average annual rainfall of 740mm; Buenos Aires 980mm and Washington DC 1000mm.

Baguio is vulnerable to moist westerly winds, such as those produced by Koppu, as it is both close to the coast and it lies at an elevation of more than 1500m.

Such deluges are not unknown. In excess of 2,200mm fell on Baguio in four days during a tropical cyclone in 1911. Typhoon Parma, locally known as Pepeng, dumped 1850mm on Baguio in one week in October 2009. More recently, in August this year more than 700mm fell as Typhoon Goni approached the island.

Computer forecasts suggest the eventual total precipitation from Koppu will be around 1250mm for some mountainous locations in northwestern Luzon.

Yet even this total is well short of some of the greatest rainfall events ever recorded.

The most remarkable rainfall event in recorded history is surely the 24 hour total of 1,825mm which fell on the Indian Ocean island of La Reunion on January 7 to 8 1966. That’s more rain than Kolkata, India receives in a year.

Another cyclone hung around the island long enough between February 24 to 27 2007 to give a 72 hour total of 3,930mm, and a 96 hour total of 4,936mm. As with Koppu, this was a slow-moving cyclone feeding off the very warm ocean waters.

Although Koppu will eventually drift northwards towards Taiwan in the coming days, the rivers across the region will continue to swell and it may be more than a week before there is any chance for normality to return to Luzon.