Beginning as a tropical depression on July 31 in the South China Sea, Sinlaku was the fifth storm of the 2020 Pacific typhoon season.

It gradually organized as it took a slow west-northwest course, strengthening into a tropical storm the following day despite its monsoonal structure.

In late July 2020, an area of atmospheric convection began to persist approximately 410 km (255 mi) east of Virac, Catanduanes.

[2] Although situated within an environment generally conducive for the formation of a tropical cyclone,[2] the low-pressure area moved towards Luzon, emerging in the South China Sea with little to no organization.

Vietnam prime minister Nguyễn Xuân Phúc urged relevant ministries and agencies "to remain updated on the developments of the storm, and map out scenarios for the possible risks of flashfloods, landsides, and severe flooding.

[14] The Copernicus emergency satellite mapping service was activated on 3 August to support the damage assessment across northern Vietnam.