The National Centre for Hydro-Meteorological Forecasting said the low-pressure system formed along the intertropical convergence zone on September 9.
It has an 80 per cent chance of developing into a tropical depression over the central East Sea in the next one to two days and a 30-40 per cent chance of strengthening further into a storm.
The system, combined with the cold spell, the intertropical convergence zone and a strong south-west monsoon, is expected to bring widespread heavy rain to several parts of the country.
From September 10 to 11, the Central Highlands and southern Vietnam are forecast to receive 60-120 mm of rain, with more than 180 mm possible in some areas.
Areas from Thanh Hoa to Hue could see 40-100 mm, locally exceeding 150 mm, while the south-central coast is forecast to receive 15-30 mm, with some areas seeing more than 80 mm.
Widespread moderate to heavy rain could persist across central Vietnam from September 12 to 17, bringing risks of tornadoes, lightning, flash floods, landslides and localised flooding.
Forecasters said the location and intensity of the rainfall could change depending on the development of the low-pressure system.
Meanwhile, a cold air mass is moving towards northern Vietnam and is expected to reach mountainous areas early on September 10 before spreading to the northern midlands and plains and north-central Vietnam later in the day.
The cold air is forecast to strengthen during the night of September 10 and into September 11, bringing cooler conditions to north-eastern and north-central Vietnam.
Temperatures are expected to fall to 21-24 degrees Celcius across much of northern and north-central Vietnam, and below 19 degrees Celcius in some high mountainous areas.
Northern Vietnam is also forecast to see scattered showers and thunderstorms from the night of September 9 through September 10, with rainfall of 10-30 mm and more than 70 mm in some areas.
Hanoi is expected to see rain and thunderstorms during the same period, with heavy rain possible in some areas, before temperatures turn cooler from September 10.
Forecasters said this is the first cold spell to affect Vietnam this year, arriving about three to five days earlier than the long-term average.
