As of 7am on August 21, a tropical depression was located over the northern Gulf of Tonkin, approximately 100km east of Bach Long Vi Special Zone. The depression is exhibiting maximum sustained winds of Force 7, or 50-61 km/h, with gusts reaching Force 9. Meteorologists indicate that the system is moving northwest at about 5 km/h and is likely to strengthen into Storm No. 4, which would be the fourth storm to impact Vietnam this year.

Once the storm enters the Gulf of Tonkin, it is anticipated to reach winds of Force 8 and gusts of Force 10. The slow movement of the storm, expected to reduce to 3-5 km/h, raises concerns for prolonged heavy rainfall across northeastern Vietnam. By 7am on August 22, the center of the storm is forecasted to be about 190km east of Hai Phong and 120km northeast of Bach Long Vi Special Zone, maintaining its intensity.

During the following 24 hours, Storm No. 4 is expected to change direction and move westward at the same slow pace. By 7am on August 23, its center is projected to remain over the northern Gulf of Tonkin, around 150km east of Hai Phong. A Level 3 natural disaster risk warning will be issued for the Gulf of Tonkin, including areas such as Bach Long Vi, Van Don, Co To, and Cat Hai special zones, as well as Hon Dau Island.

In terms of rainfall, the combined effects of the intertropical convergence zone have already resulted in moderate to heavy rain across northeastern Vietnam and areas from Thanh Hoa to Nghe An. Rainfall from 7pm on August 20 to 8am on August 21 exceeded 100mm at several monitoring stations, including Thanh Ha in Hanoi, which recorded 122.8mm. From August 21 through the night of August 22, moderate to heavy rain and thunderstorms are expected to continue, with widespread totals of 70-150mm and isolated areas potentially receiving over 300mm.

Authorities have warned that rainfall intensity could exceed 120mm within just three hours. Nghe An province is also forecasted to experience moderate to heavy rain, with totals of 20-40mm expected. Meteorological officials have advised residents in northeastern provinces and northern mountainous areas to prepare for the risk of flooding and landslides as the wet conditions may persist.