Typhoon Ketsana Is Formed

Typhoon Ketsana (International designation: 0916, JTWC designation: 17W, PAGASA name: Ondoy) formed early on September 26, 2009, about 860 km (535 mi) to the northwest of Palau.

The depression remained weak and was downgraded to a low pressure area later that day by the Japan Meteorological Agency. The low pressure area then reintensified early the next day and was named as Tropical Depression Ondoy by the Philippine Atmospheric, Geophysical and Astronomical Services Administration as the Joint Typhoon Warning Center issued a Tropical Cyclone Formation Alert on the depression. It was then reupgraded to a tropical depression by the JMA later that morning before the JTWC followed suit early on September 25, designating the depression as 17W. The intensification of Ondoy was hampered throughout September 25 by the system moving into an area of moderate vertical windshear and an upper level trough of pressure which was moving over the system. It was then upgraded to a Tropical Storm and named as Ketsana before passing over the island of Luzon in the Philippines. As it moved into the South China sea later that day the system dramatically deepened and expanded whilst moving towards the west which lead to the JMA upgrading it to a Severe Tropical Storm.

On September 23, 2009, the Japan Meteorological Agency (JMA), reported that a tropical depression of the season had formed about 860 km (535 mi) to the northwest of Palau. The Joint Typhoon Warning Center (JTWC) then reported later that day that the depression had a developing low level circulation center and was in an favourable environment with low vertical wind shear. The JMA then reported later that day that the depression had weakened into an area of low pressure. 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 to the depression. A Tropical Cyclone Formation Alert was then issued later that morning by the JTWC as central convection had continued to organize around a consolidating elongated but exposed low level circulation center. The JMA then reupgraded Ondoy to a tropical depression later that morning before the JTWC followed suit early on September 25, designating it as Tropical Depression 17W whilst located about 400 nm to the east of Manila in the Philippines. The system at this stage was moving along the southern side of the subtropical ridge and had good poleward outflow into a Tropical Upper Tropospheric Trough (TUTT) cell.