Forming out of an area of low pressure on July 9, Soudelor failed to maintain deep convection around its center for the duration of its existence.

The storm later killed 15 people in southern China after a group of hikers were washed away in a flash flood on Hainan Island.

In Vietnam, rainfall up to 250 mm (9.8 in) caused widespread flooding, and lightning triggered by the storm killed two people in the country.

Late on 7 July 2009, the Joint Typhoon Warning Center (JTWC) reported that an area of disturbed weather had formed 900 km (560 mi) to the northwest of Yap.

Deep convection was embedded in a broad, weak, poorly defined circulation that was starting to be enhanced by a tropical upper tropospheric trough to the east of the system.

[1] Over the next couple of days, gradual development took place[2] and early on July 9, a Tropical Cyclone Formation Alert was issued by the JTWC.

[3] Around the same time, the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) designated the system as a tropical depression, giving it the name Gorio.

[6][9] Later that day the JTWC reported that Soudelor had weakened into a depression;[10] however they re-upgraded it to a tropical storm as it moved closer to Hainan Province.

[15] As a tropical depression, Soudelor brushed northern Luzon in the Philippines, producing upwards of 330 mm (13 in) of rainfall which resulted in flash flooding and landslides.

[17] Following the storm, the hardest hit town Bacarra, Ilocos Norte, was put under a state of calamity and regional aid was deployed to the area.

Roughly VND71 billion (US$3.98 million) was set aside by the Government of Vietnam to assist residents in natural disasters, including Soudelor.