Thailand's Meteorological Department (TMD) has issued a new forecast warning that, driven by a persistent monsoon trough, scattered heavy to very heavy rain will affect multiple regions of the country from September 17 to 20. According to the forecast, the North and Northeast will be the first regions hit by heavy rain on September 17, before the rain band expands to the Central region, the East and the South from September 18 onward. TMD specifically named Lampang, Phayao, Nan and Phrae in the North, along with Nong Khai in the Northeast, as areas likely to see very heavy rain, with an elevated risk of flash floods and forest runoff. The department also forecast strengthening winds over the Andaman Sea from September 17-20, with waves of 2-3 metres and potentially exceeding 3 metres in thunderstorm areas, advising small boats to remain ashore during this period.

In response to the coming heavy rain, Thailand's Department of Disaster Prevention and Mitigation (DDPM) has alerted all provinces nationwide, including Bangkok, to prepare for the period of September 16-20, warning of risks including flash floods, forest runoff, landslides, waterlogging in communities and low-lying areas, and rougher seas along the coast. Separately, the Office of the National Water Resources (ONWR) has warned that 28 provinces across five regions of the country face flood and flash-flood risk from September 15-19, urging local governments and residents to prepare drainage systems, evacuation plans and the advance movement of belongings. The breadth and duration of this warning underline how widespread the current rain system is expected to be.

Among the affected areas, the situation in Phetchabun province, particularly Lom Sak Municipality, has been especially tense. According to the provincial disaster prevention office, the Pa Sak River running through the province rose to 7.67 metres on September 15 and continued climbing, prompting the Phetchabun DDPM office to issue a flood warning and send an emergency Cell Broadcast alert, in coordination with mobile operators AIS, True and National Telecom, to residents living along the river in Tat Kloi subdistrict, Lom Kao district. Residents were told to move belongings and vehicles to safety immediately and to prioritise helping vulnerable people relocate to higher ground. At the time, the water level at monitoring station S.33 in Tat Kloi, Lom Kao district, was already close to the overflow point, and authorities warned that riverside communities, tributary areas and low-lying neighbourhoods faced a high risk of inundation.

By September 16, the situation had escalated further. At 8am that day, the Pa Sak River at monitoring station S3 in Lom Sak Municipality rose to 8.12 metres and was still climbing, marking the highest water level ever recorded at that station. Lom Sak Municipality immediately raised its highest-level flood warning. Meanwhile, upstream stations showed a different trend: station S33 in Lom Kao district stood at 5.97 metres and falling, while station S53 near Ban Wang Rong Bridge read 4.55 metres, also declining. Disaster officials noted, however, that even as upstream levels eased, runoff accumulated from days of heavy rain in upstream areas and tributaries was still converging downstream toward Lom Sak, and combined with the municipality's limited drainage capacity, this kept flood pressure high in the town. Authorities warned that continued rainfall and inflow into the Pa Sak River could push water levels even higher, urging residents in low-lying and riverside areas to remain vigilant, move valuables, vehicles and livestock to safety as soon as possible, and follow local evacuation orders.

Phetchabun and communities along the Pa Sak River have repeatedly faced severe flooding in recent years. In 2025, Tropical Storm Nongfa brought the worst flooding in more than a decade to Phetchabun; at the time, a monitoring station in Tat Kloi subdistrict, Lom Kao district, recorded an extreme level of 12.6 metres, a record at the time, before the sensor itself was submerged and readings were interrupted. The volume and force of that floodwater exceeded any previous event. Several roads in Lom Sak Municipality were left under as much as 80 centimetres of water, with government buildings and homes affected on a wide scale. That the Pa Sak River at the S3 station in Lom Sak has now set a new record again this year underscores the structural flood-control pressure this river basin repeatedly faces during the rainy season, and highlights the urgency of upstream soil and water conservation and integrated basin management.

Beyond Phetchabun, other parts of Thailand face varying degrees of flood risk. Nakhon Ratchasima has issued a warning over the risk of reservoirs overflowing in its area, while four districts in Nan province have been placed on a watch list for heavy to very heavy rain, with local disaster authorities issuing corresponding flood warnings. Thai media reports indicate that since the start of this year's rainy season, parts of the North have already experienced sustained heavy rain and multiple flood events, with flooding in some provinces yet to fully recede. Combined with this new round of heavy rain forecast for September 17-20, these areas could face renewed waterlogging or flooding again in the near term.

TMD and the DDPM have reminded the public to pay particular attention to the safety of hillside areas, riverbanks and low-lying zones during periods of heavy rain and flood warnings, cautioning against sudden risks such as flash floods, landslides and rapidly rising river levels, and urging people to closely follow the latest official warnings issued via radio, SMS and Cell Broadcast systems, and to comply with local evacuation orders. Analysts note that extreme rainfall events in Thailand have become more frequent and more intense in recent years, and that drainage infrastructure in some river basins has lagged behind, leaving areas such as Phetchabun to repeatedly confront record-breaking water levels. The new record set by the Pa Sak River at Lom Sak is not only a real test of local flood-response capacity, but also raises fresh questions for Thailand on strengthening basin management and early-warning systems amid a changing climate.