Usagi strengthened to hurricane intensity as it approached Vietnam from the South China Sea but weakened to tropical storm intensity when coming ashore. Very heavy rainfall and damaging winds accompanied tropical storm Usagi when it hit Vietnam's southern coast. More than 350 mm (14 inches) of rainfall was reported causing widespread flooding around Ho Chi Minh City.

NASA's Integrated Multi-satellitE Retrievals for GPM (IMERG) data were used to show estimates of rainfall accumulation produced by Usagi as the tropical cyclone moved across the South China Sea into Southeast Asia. This IMERG accumulation analysis encompassed the period from November 19-26, 2018. Those data indicated that Usagi dropped moderate to heavy rain along it's path. This analysis shows that tropical storm Usagi often dropped more than 240 mm (9.4 inches) of precipitation over southern Vietnam. Rainfall intensity decreased rapidly as the remnants of Usagi moved inland over Cambodia.