On December 11, a disturbance formed and stayed near Chuuk. On December 13, the low pressure area quickly got stronger. On the same day, the JTWC upgraded the low pressure to a tropical depression. They named it "27W". The JMA also upgraded it to a tropical depression. The JTWC upgraded the system to a tropical storm on December 14. They then downgraded it to a tropical depression early on December 15. The PAGASA called 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. They named it "Washi". On December 16, Washi made landfall over Surigao del Sur, a province of the Philippines. Several hours later, Washi arrived at the Sulu Sea and regained its strength quickly. Late on December 17, Washi crossed Palawan, and arrived at the South China Sea. On December 19, Washi weakened into a tropical depression and dissipated.