Regarding the case “Heavy to very heavy rain” in many areas across Thailand from 23 to 27 September 2026, recently causing flooding in several parts of Bangkok. The accumulated rainfall from 24 Sep 2026 at 16:00 until this day (26 Sep 2026) at 13:00 has exceeded 300 millimeters. Consequently, Bangkok declared a disaster zone to facilitate public relief efforts.
Today (26 Sep), Chatchart Sittipunt, Governor of Bangkok, revealed that meteorological department models predict rainfall will gradually ease. If this forecast holds, water drainage will become more efficient. Assuming no further rain adds to the system, drainage could take about 2-3 days. The main issue is many canals have critically high water levels flooding communities, but along the Chao Phraya River where Bangkok has strong pumping capacity, the situation is not yet critical.
When flooding occurred in many Bangkok areas, residents noted that some locations had never flooded before. Additionally, online posts circulated warning of a possible “great flood disaster” causing widespread public concern whether this Bangkok flood could repeat the major 2011 flood.
Thanaroj Waratsprasert, Director of the National Water Command Center (NWCC), Office of National Water Resources (ONWR), told Thairath Online's special news team on 23 September that the current situation is less severe than the 2011 flood and does not constitute a "great flood disaster." A great flood occurs when large water masses flow from mountainous areas or the north, overwhelming management efforts, causing water levels 2-3 meters high. However, the current condition involves rain falling directly on flat areas, resulting in flooding from overflow waiting to drain gradually into rivers and eventually to the sea.
Similarly, on 24 September, Asst. Prof. Dr. Thon Thamrongnawasawat, a marine and environmental academic at Kasetsart University's Faculty of Fisheries, stated that the possibility of a slow-moving great flood inundating provinces as in 2011 is nearly impossible, citing these differences:
1. Different weather phenomena and rainfall amounts.
In 2011, during the first half of the year, there was a “La Niña” phenomenon causing above-average rainfall starting early in March-April. La Niña also brought five tropical cyclones—depressions, tropical storms, and typhoons—directly affecting Thailand.
In contrast, in 2026, the region is experiencing “El Niño” which brings below-average rainfall and is entering a super El Niño phase. This year, no tropical cyclones have directly impacted Thailand.
2. Flooded areas.
Comparing flooded areas in August (as September is incomplete) of both years, GISTDA satellite data reports 5.7 million rai flooded in August 2011, versus less than 400,000 rai in August 2026, a difference of multiple times less.
Dr. Thon explained that heavy, localized rainfall accumulating 100-200 millimeters per spot causes flash floods or ponding typical of the rainy season, especially September to October, the heaviest rain period, even during super El Niño.
However, current spot rainfall is more intense due to global warming, as higher sea evaporation increases atmospheric moisture by about 7% per 1°C temperature rise, meaning “exceptionally saturated skies.” Thus, localized heavy rain often causes sudden flooding and high ponding in certain areas, especially those geographically prone to poor drainage, such as Pattaya city, several Bangkok districts, and “large water volumes” that flow and flood places like Phra Nakhon Si Ayutthaya and Prachinburi provinces.
Meanwhile, Assoc. Prof. Dr. Seri Suparat, Director of the Climate Change and Disaster Center at Rangsit University and Vice President of the National Disaster Warning Council Foundation, said the rainfall amount in Bangkok is abnormally high, exceeding 300 mm accumulated, while many Bangkok areas flood with as little as 60 mm. Such heavy accumulated rain causes widespread flooding.
Dr. Seri noted the severe flooding partly results from a lack of risk assessment on rainfall severity. Local authorities should evaluate which areas would flood if rainfall surpasses 300 mm to prepare warnings and deploy sufficient pumps effectively.
Nonetheless, he urged people not to panic, emphasizing this event is unlike the 2011 great flood involving massive northern water flows taking up to three months to resolve. Current floods come from heavy local rainfall causing standing water and canal overflows. If drainage is effective, flooding may last no more than one week. He advised close monitoring and avoiding travel to prevent vehicle damage.
National Water Data Repository states that in 2011, rain came early and was above average nearly every month starting March, totaling 1,826 millimeters for the year—25% above normal.
Rainfall distribution covered a wider area than usual, especially northern Thailand, causing massive water flows toward central regions. That year, Thailand was hit by five storms: tropical storms Nock-ten, Haima, Haitang, and typhoons Nesat and Nalgae. Though these weakened before landfall, they increased rainfall. The monsoon trough also passed over Thailand from May to November, combined with La Niña increasing rainfall. La Niña phenomenon caused higher rainfall trends in Thailand.
Meanwhile, in 2026, the world faces an “El Niño” phenomenon causing drought risk and below-average rainfall. The Meteorological Department forecasts national rainy season rainfall to be lower than in 2025 and about 10% below average. August to September will have concentrated rain requiring vigilance for heavy rain, flash floods, flash floods, and river overflows in many areas.
Dr. Thon noted that although it is a super El Niño year, global warming intensifies localized rainfall, increasing the risk of severe flooding.
Regarding water discharge from the Chao Phraya Dam, a key control point for northern to central water flow, as of 26 September, discharge has been increased from 1,850 cubic meters per second to 1,950 cubic meters per second, since 09:00 today.
Thanaroj Waratsprasert, NWCC Director, previously stated that this level is a warning threshold requiring close monitoring, but if discharge remains below 2,000 cubic meters per second, it is still manageable.
Comparing to 2011, data from station C.13 behind the Chao Phraya Dam in Sapphaya District, Chai Nat Province, showed peak flow reaching 3,721 cubic meters per second on 21 September 2011, with total flow from 1 April to 18 November reaching 31,762 million cubic meters.
During the 2011 flood, dams across the country held record-high water volumes but could not fully discharge effectively. Of 33 large dams, 21 experienced overflow beyond normal storage levels.
Total annual inflow to dams reached 71,769 million cubic meters, the highest on record, especially at Bhumibol, Sirikit, and Pasak Jolasid dams, which critically affected flooding in northern and central regions.
As of 26 September 2026, according to the Royal Irrigation Department, accumulated inflow since 1 January 2026 is 30,675 million cubic meters. Bhumibol Dam holds 8,437.68 million cubic meters (63% capacity), and Sirikit Dam holds 7,592.12 million cubic meters (80% capacity), with both dams having nearly 7 billion cubic meters of available storage.