On December 11, a disturbance formed and persisted near Chuuk. On December 13, the low pressure area rapidly intensified prompting the JTWC to issue a TCFA. On the same day, the JTWC upgraded the low pressure to a tropical depression and designated as 27W; in addition, the JMA also upgraded it to a tropical depression. The JTWC upgraded the system to a tropical storm on December 14 but downgraded it to a tropical depression early on December 15, and the PAGASA designated it Sendong as it entered the Philippine Area of Responsibility. After passing Palau on December 15, both the JTWC and the JMA upgraded the system to a tropical storm and named it Washi. On December 16, Washi made landfall over Surigao del Sur, a province of the Philippines located in Mindanao. Several hours later, Washi arrived the Sulu Sea and regained its strength quickly due to slight land interaction with Mindanao. Late on December 17, Washi crossed Palawan and arrived the South China Sea. On December 19, Washi weakened to a tropical depression and dissipated.
In the Philippines, Washi has caused at least 1,257 fatalities, and 1,079 people are officially listed as missing. Washi had affected 102,899 families or 674,472 people in 766 villages in 52 towns and eight cities in 13 provinces. The majority of the deaths were in the cities of Iligan and Cagayan de Oro. Five people were killed in a landslide, but all others died in flash flooding. More than 2,000 have been rescued, according to the Armed Forces of the Philippines. Officials were also investigating reports that an entire village was swept away. The flash flooding occurred overnight, following 10 hours of rain, compounded by overflowing rivers and tributaries. In some areas, up to 20 centimeters of rain fell in 24 hours. At least 20,000 people were staying in 10 evacuation centers in Cagayan de Oro. Officials said that despite government warning, some people did not evacuate. At least 9,433 houses were destroyed while 18,616 were damaged