Fujiwhara effect
On August 25, a low-pressure area developed in the South China Sea, west of the Philippines, associated with the dissipating convection of Tropical Storm Zita. The low-pressure area then started to strengthen, and slowly became a Tropical Depression on August 28. The system failed to strengthen for a while, due to a much stronger system just east of Cass, Typhoon Amber. However, Typhoon Amber soon started to move north, just a little farther from Cass, but was just enough for Cass to strengthen into a Tropical Storm. As the nearby Typhoon Amber started to make landfall over Taiwan, a strong southern inflow from Typhoon Amber pulled Cass northwards, in an effect known as the Fujiwhara effect. As Typhoon Amber made landfall, and moved inland over eastern China, Cass was pulled very close to Typhoon Amber, while Cass was maintaining Tropical Storm strength. This severely weakened the storm, as it made landfall on August 30. Cass was then absorbed by Typhoon Amber, shortly afterwards, in inland China.