TMD watches Vietnam low as two Thai provinces face very heavy rain risk on Sept 14

The Thai Meteorological Department (TMD) is monitoring a strong low-pressure area over Vietnam which, in its September 13, 2026 assessment, had not developed into a storm and was not expected to enter Thailand directly. The department’s September 14–15 forecast warns of isolated very heavy rain in two provinces, Ubon Ratchathani and Trat.

The TMD issued its clarification after social media posts about a possible new storm over upper Vietnam raised concerns about a direct impact on Thailand. The department said the system still required close monitoring because further development remained possible.

TMD sees low tropical-storm odds for Vietnam system

The TMD’s September 13 analysis put the chance of the Vietnam system becoming a tropical storm at a low level, citing modelling from the European Centre for Medium-Range Weather Forecasts (ECMWF). The department set out four possible outcomes:

1. Remaining a strong low-pressure area: high likelihood.
2. Developing into a tropical depression: possible, but with limited likelihood.
3. Developing into a tropical storm: low likelihood.
4. Developing into a typhoon or major storm: very low likelihood.

The TMD cautioned that the ECMWF probabilities came from ensemble modelling, which uses multiple forecast simulations, and did not confirm that a storm would form. The analysis included MAZU–FengYun satellite imagery timestamped 5pm on September 13.

The TMD identified five factors limiting development of the Vietnam low-pressure system:

1. Proximity to land: the system could not draw energy continuously from the sea and would weaken near the coast or over land.
2. Mountainous terrain in Vietnam and Laos: high ground could disrupt winds and circulation, weakening or splitting the centre.
3. An interrupted moisture supply: moist marine and monsoon air was not feeding the centre strongly and continuously.
4. Cooler, drier air from the north and east: this reduced moisture and prevented rain clouds from forming symmetrically around the centre.
5. Disorganised cloud structure: satellite imagery showed a broad rain-cloud mass without an organised tropical cyclone structure.

The TMD said subsequent ECMWF runs should be monitored for changes in cyclone probabilities, ensemble tracks, minimum central pressure, and moisture and wind patterns at the 850-hectopascal atmospheric level.

Vietnam low could strengthen Thailand’s monsoon

The TMD said the Vietnam low-pressure system could strengthen the monsoon affecting Thailand as it moved into Laos and weakened. The indirect effects could bring more cloud, increased rainfall and isolated heavy downpours despite the absence of an expected direct track into Thailand.

The TMD’s September 13 assessment outlined different potential effects across the Northeast:

• Upper Northeast: cloudy conditions, scattered light to moderate rain and periods of continuing rainfall.
• Central Northeast: intermittent rain, with thunderstorms possible in some areas.
• Lower Northeast: scattered rain, depending on the system’s centre and the rain bands to its south.

For September 14–15, the TMD described a monsoon trough extending across the upper South, East and lower Northeast into a strong low-pressure area over central Vietnam. A moderate south-westerly monsoon also covered the lower Andaman Sea, lower South and lower Gulf of Thailand.

The TMD warned that heavy or very heavy rainfall and accumulated rain could trigger flash floods, forest run-off, mudslides and localised waterlogging. People near foothills and waterways, and in low-lying areas, were advised to remain alert and avoid routes prone to repeated flooding.

The TMD said it would continue monitoring changes in the Vietnam weather system and urged the public to follow its forecasts and announcements regularly because the system’s direction and intensity could change.