It continued to move northwest over the warm waters of the Gulf of Tonkin, eventually making landfall in Vietnam over Thanh Hóa by the end of the same day, rapidly weakening afterward.

At 8:30 UTC on June 10, the Joint Typhoon Warning Center (JTWC)[nb 1] started to monitor a low-pressure area in the South China Sea, located approximately 518 km (322 mi) to the south of Hong Kong, with the agency classifying the system as a monsoon depression as it was embedded within a monsoon trough that extends from Laos to the Philippines.

[2][3][4] Tracking west-northwestward, the storm was located in a favorable environment for tropical cyclogenesis, with warm sea surface temperatures up to 30–31 °C (86–88 °F) and low wind shear, being offset by the lack of divergence aloft in the area.

[6][7] An anticyclone over China started to move eastward at this time, increasing the divergent flow on the area which allowed slight intensification of the system.

[10] Remaining weak, Koguma moved northwestward and subsequently made landfall over north-central Thanh Hóa at 18:00 UTC on the same day, at peak intensity with winds of 65 km/h (40 mph) and a barometric pressure of 996 mbar.

[14] At 15:45 UTC on June 13, the JMA also issued the final warning on Koguma as it weakened further to a tropical depression and subsequently dissipated over the terrains of Vietnam.

[15] As Koguma approached Vietnam from the Gulf of Tonkin, the city of Thái Bình ordered the suspension of fishing activities in the area due to the forecasted rough waters from the storm.

[16] The People's Committee of the city, on the other hand, urged the individuals in the area to cut the trees that would disrupt major roadways and put support on their houses, warehouses, schools, hospitals, aquaculture and fishing farms to prevent any damages.

[21] The Ministry of Water Resources in Cambodia raised an alert warning on the country due to the possible effects of Koguma, being influenced by the prevailing southwest monsoon.

[23] The Thai Meteorological Department noted that moderate to heavy rainfall were expected across some places in Northern and Northeastern portions of Thailand due to the influence of the monsoon trough and the remnants of the system.

[27] 2 people in Thanh Hóa's coast were reported missing,[28][29] while an individual was confirmed dead in Yên Bái Province on June 14 by the Vietnam Disaster Management Authority due to Koguma.