The depression remained weak and was downgraded to a low pressure area later that day by the Japan Meteorological Agency (JMA) but after drifting through extremely favorable conditions, it intensified the next day and was categorized as Tropical Depression by the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) and was given the name Ondoy after entering the Philippine Area of Responsibility.

President Gloria Macapagal Arroyo declared a "state of calamity" encompassing most of Luzon after at least 86 people were initially reported dead in landslides and other incidents.

[a] On September 23, 2009, the Japan Meteorological Agency (JMA), reported that a seasonal tropical depression had formed about 860 km (530 mi) to the northwest of Palau.

[12] However, early the next day, as deep convection started to consolidate around the low-level circulation center, the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) reported that the low-pressure area had become a tropical depression and assigned it a local name of Ondoy.

[13][14] Later that morning, the JTWC issued a Tropical Cyclone Formation Alert as central convection had continued to organize around a consolidating elongated but exposed low-level circulation center.

[16][17] At this stage, the system was moving along the southern side of the subtropical ridge and had good poleward outflow into a tropical upper tropospheric trough (TUTT) cell.

During September 27, Ketsana gradually developed further and was upgraded to a typhoon by the JTWC and the JMA early the next day, as multiple convective bands were continuing to consolidate more tightly around the low-level circulation center, leading to the formation of a disorganized eye.

[21] On September 28, PAGASA lifted all public storm signals in the country as Ketsana left the Philippine Area of Responsibility (PAR) the same day.

[23] On September 29 it was announced that parts of southern China would be placed under an orange warning with certain regional meteorological bureaus entering a level 3 emergency response.

On September 27, the Vietnam National Center for Hydro-Meteorological Forecasting[25] issued a public storm warning signal named "Number 9."

Also, authorities mobilized several thousand military personnel and police to help residents evacuate from the typhoon's path.

Ketsana caused widespread flash flooding in the cities of Manila, Caloocan, Marikina, Malabon, Muntinlupa, Quezon, Makati, Pasay, Pasig, Taguig, Valenzuela, and San Juan.

Defense Secretary and National Disaster Risk Reduction and Management Council (NDCC) chairman Gilbert Teodoro asked the DOTC to keep MRT and LRT lines operational to accommodate stranded passengers.

[29]On the afternoon of September 26, Gilbert Teodoro declared an overall state of calamity in Metro Manila and the nearby 25 provinces in Luzon hit by the typhoon, allowing officials to utilize emergency funds for relief and rescue.

Marikina, part of Metro Manila, was the most devastated region in the Philippines: almost all of the city's area was submerged in water up to ten feet deep and tons of knee-deep mud.

[39] Over two days starting on September 29, the National Power Corporation Flood Forecasting and Warning System released 500 cubic meters per second of stored water from the Angat Dam in Bulacan.

[citation needed] Heavy rains and strong winds lashed a 400 km (250 mi) stretch of coastline from Thừa Thiên–Huế to Quảng Ngãi, with rainfall causing massive flood surges in Huế, Bình Định, and Kon Tum provinces.

[51] Ketsana also injured one person and killed two before moving out of the country as an area of low pressure and dissipating on October 3 over the Andaman Sea.

[51] Three dams in Chai-ya-poom were damaged by the heavy rainfall, while in Pattaya nine boats were sunk waves reported to be over two metres high.

[52] The Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) documented a record-high amount of rainfall in 24 hours at 455 millimetres (17.9 in).

[56] Some 48 hours after Ketsana struck Metro Manila, the Philippine government appealed to the international community and the United Nations for help.

[62] The United States also deployed Marines to help rescue victims in the Cainta and Pasig areas, as well as for search and retrieval operations for dead bodies.

Nonprofit international disaster relief organization AmeriCares shipped $3.2 million of medical aid for Ketsana survivors.

[69] In the Philippines, the National Disaster Coordinating Council (NDCC) headed the rescue and relief operations for the citizens affected by Ketsana's flooding.

[8][71] On the Internet, citizens turned to social networks like Twitter, Facebook, Plurk, and Multiply to share news updates and forward cries for help from people trapped in the floods.

Google Maps was used to pinpoint the locations of stranded people, while various blogs and websites shared information on donating money and in-kind goods.

Nguyen Viet Thang, chairman of the Vietnam Fishery Association, said his organization was preparing an official protest to China over the incidents.

When President Benigno Aquino III took office in June 2010, PAGASA Chief Administrator Prisco Nilo was fired and removed from his post on August 6.