According to the National Disaster Risk Reduction and Management Council (NDRRMC), Tropical Storm Sendong entered the Philippine Area of Responsibility as a tropical depression on 15 December 2011 and shortly intensified into a tropical storm. As it crossed the Philippines, the storm affected seven regions: IV-B (MIMAROPA), VII (Central Visayas), IX (Zamboanga Peninsula), X (Northern Mindanao), XI (Davao), XIII (Caraga) and ARMM (Autonomous Region in Muslim Mindanao). Region X includes the most heavily affected areas of Cagayan de Oro City and Iligan City.
The Cagayan, Agus and Mandulog rivers rose rapidly in the early hours of 17 December, with fast flowing muddy waters surging over riverbanks and sweeping away buildings from a swathe of land on either side. The rivers’ rapid speed and rise - in some areas rising by 3.3 meters in less than an hour - caused devastation more commonly seen from tsunamis, with entire neighborhoods and villages swept away. The flash floods struck in the early hours of the morning, giving residents little warning and killing many people as they slept.
NDRRMC reports, as of 24 January 2012, a total of 1,144,299 affected persons. Specifically, around 81,910 people are displaced – within the capacity of evacuation centers. The remaining 441,022 displaced persons remain in makeshift shelters, with host families, renting of properties, or without access to any shelter. A total of 52,435 houses were damaged (14,883 totally and 37,552 partially)