According to the National Centre for Hydro-Meteorological Forecasting, the centre put the probability of the system developing into a storm at 30-40 per cent.
A tropical convergence zone is expected to form over the East Sea from September 9, with its axis extending across central Vietnam.
Meanwhile, a weak cold spell has a 70-80 per cent chance of affecting northern Vietnam between September 9 and 11, bringing rain, thunderstorms and lower temperatures.
Northeasterly winds over the northern Gulf of Tonkin are forecast to reach levels 5-6, with waves of 2-3 metres and rough seas.
Temperatures in the north are expected to fall below 25 degrees Celsius at night and 33 degrees during the day, while lows in mountainous and midland areas could drop below 23 degrees.
The centre said this would be the first cold spell of the 2026-2027 winter season and would arrive around three to five days earlier than the long-term average.
The combination of the tropical system, cold air and convergence zone could bring widespread moderate to heavy rain to central Vietnam from around September 10-17, raising the risk of flash floods, landslides and localised flooding.
Under the more likely scenario, with a probability of 60-70 per cent, heavy rain would begin on September 10 and last through September 16, with the heaviest rainfall expected from Ha Tinh to Gia Lai.
A second scenario, with a 30-40 per cent probability, would see heavy rain from around September 14-17, concentrated between Nghe An and Hue.
The southwest monsoon is also expected to strengthen from September 9-12, bringing more thunderstorms to the Central Highlands and southern Vietnam.
In the shorter term, areas from Quang Tri to Danang, eastern Quang Ngai and the Central Highlands were forecast to receive 10-30 mm of rain on September 7, with isolated totals exceeding 70 mm.
Eastern southern Vietnam could receive 10-20 mm, with more than 50 mm possible in some areas.
Thunderstorms could bring tornadoes, lightning, hail and strong gusts, while intense localised rainfall could trigger flash floods, landslides and flooding in low-lying areas, forecasters warned.