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Tropical Storm Washi Triggers Disastrous Flooding in Philippines

The death toll from Tropical Storm Washi, which struck the southern island of Mindanao in the Philippines over the weekend, has reached at least 900 and climbing. The storm may soon become the deadliest storm of 2011, beating out flash floods in Brazil in January.

Although the Philippines overall is a country that is no stranger to tropical cyclones, these storms typically bypass the southernmost areas, which means that Washi struck a relatively unprepared population.

NASA satellite view of Tropical Storm Washi as the storm approached the island of Mindanao in the Philippines on Saturday.

Washi made landfall on Saturday night as a minimal tropical storm with 45 mph winds, but it was embedded within a massive plume of tropical moisture that contributed to heavy rainfall on the island that lasted for as long as 100 hours. The flash flooding that resulted swept people from their homes, leaving destroyed communities in their wake. As the New York Times reports:

In neighborhoods throughout the cities of Iligan and Cagayan de Oro on the island of Mindanao in the southern Philippines, similar stories could be heard: Flash floods caused by Tropical Storm Washi surged into the homes of sleeping families, and hundreds were drowned or dragged to their deaths by the currents.

The storm hit an area in the Philippines that is not accustomed to tropical storms and typhoons, since such storms typically form and make landfall to the north of the island of Mindanao. This unfamiliarity with such storms  heightened the risks of a mass casualty event.

Over at Weather Underground, Jeff Masters notes the presence of unusually warm sea surface temperatures offshore, which contributed to the heavy rainfall.

"Aiding the heavy rains were sea surface temperatures that were nearly 1°C above average off the east coast of Mindanao, one of the top five warmest values on record. The exceptionally warm waters added about 7 percent more moisture than is usual for this time of year to the atmosphere," Masters writes.

Map of the tracks of tropical cyclones, with an arrow and circle pointing to the location of the Tropical Storm Washi disaster. Credit: NASA.

Masters also points out that the rains fell on regions that have experienced unsustainable development, such as illegal logging and lands being converted to pineapple plantations. In such areas, "...the heavy rains were able to run off quickly on the relatively barren soils and create devastating flash floods. Since the storm hit in the middle of the night, and affected an unprepared population that had no flood warning system in place, the death toll was tragically high." 

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