The Southern Regional Hydro-Meteorological Centre said moderate to very heavy rain was forecast across the city through October 3, with rainfall of 30-60mm and more than 70mm in some areas.
Rain is expected mainly in the afternoon and evening, with thunderstorms potentially bringing tornadoes, lightning and strong gusts. Localised downpours could cause flooding in low-lying areas and places with poor drainage.
The wet spell could persist for several days, with daily rainfall of 30-60mm and isolated totals exceeding 60mm.
A low-pressure trough is forecast to move southwards over the next three to 10 days. An intertropical convergence zone could form around October 5-6 and gradually strengthen.
Nguyen Van Tin from HCM City University of Natural Resources and Environment said the heavy rain in late September and early October resulted from several overlapping weather systems rather than typical convective showers.
Since September 30, cold air moving into northeastern Vietnam has pushed a convergence trough southwards, where easterly and southwesterly winds have combined to trigger heavy rain.
Tin said such weather patterns were not unusual for HCM City, but their simultaneous effects could increase the likelihood of intense thunderstorms and heavy rainfall.
Recent downpours have also coincided with high tides, hampering drainage and worsening flooding. Le Minh Xuan Station recorded 366.6mm of rain between September 29 and October 2, while several other areas received more than 100mm.
Rainfall across southern Vietnam in October is forecast to be 5-20% below the long-term average. However, rain is expected to be more frequent in the first and middle parts of the month before easing towards the end.
The region could see one or two heavy rain spells and one or two tropical cyclones over the East Sea during the month.
Towards late October, high tides are expected to become a greater flooding concern. Tide levels are forecast to peak at 1.7-1.75 metres at Phu An and 1.82-1.87 metres at Thu Dau Mot from October 27-29, both above the third warning level.
Residents have been advised to monitor weather warnings, particularly in the afternoon and evening, and take precautions against lightning, strong winds and localised flooding.