On the evening of September 21, at almost the same time when Nesat was first seen, another low pressure area persisted far south of Hong Kong. The low slowly drifted north and strengthened slowly until on September 24, when the JMA upgraded the system to a tropical depression east of Vietnam. Later that day, the JTWC issued a Tropical Cyclone Formation Alert stating that the low could develop into a tropical cyclone. Early on September 25, the JMA upgraded the system to a tropical storm and named it "Haitang".