Check the flood watch areas from 20 to 22 May. Which provinces need special monitoring? The Director of the National Water Command (NWC) reveals rainfall thresholds that indicate risk levels for landslides and flash floods.
Due to sudden flash floods and flash floods in several areas of Phetchabun province on 20 May 2026, 4 subdistricts and 13 villages were affected by the disaster.
Recently, Mr. Saranyu Meethongkham, Governor of Phetchabun province, ordered district chiefs and local administrative organizations to assist the affected residents and urgently survey the damage. He also warned local people to prepare and continue monitoring the situation.
Thailand has officially entered the rainy season since 15 May. The recent events in Phetchabun reflect the country's need to prepare for flooding and flash floods during this rainy season. This year, certain areas and provinces have varying levels of risk.
Thairath Online special news team inquired about this issue with Mr. Thanaroj Waratsprasert, Director of the National Water Command (NWC). He revealed that from 20 to 22 May 2026, Thailand will be affected by the monsoon storm and winds from the Andaman Sea, impacting the following provinces:
In these areas, it is forecasted that rainfall may accumulate over three days with amounts exceeding 200 millimeters. However, this is only a prediction and the situation must be continuously monitored.
“This marks the start of the rainy season, with water flowing rapidly in mountainous and sloped areas, causing drainage delays. These are typical flash floods that usually subside within a few hours,” Mr. Thanaroj said, adding that these water volumes are not expected to cause major flooding or reservoir overflows.
Regarding Bangkok, Mr. Thanaroj expects heavy rain in some areas but not continuously. Some localized flooding might occur in certain districts due to drainage system issues. After 22 May, rainfall and wind are expected to ease.
During the rainy season, rainfall typically exceeds average levels in August and September. In October, the central region must closely monitor water levels to guard against flooding, as continuous rain may overwhelm drainage capacity.
Mr. Thanaroj said studies on rainfall amounts show that in the northern region, if rainfall reaches at least 150 millimeters over 12 consecutive hours, there is a risk of flash floods and landslides. In the southern region, rainfall of 175 millimeters over a prolonged period also poses flood risks. Therefore, rainfall levels must be closely monitored to remain within safe limits.
Currently, forecasts from the Meteorological Department and the Water Resources Information Institute are used to observe rainfall amounts, combined with soil erosion potential data from the Department of Water Resources and the Department of Mineral Resources to assess risk areas.
Overall, forecasts from the Meteorological Department and the U.S. NOAA indicate a strong El Niño influence this year, resulting in rainfall amounts in Thailand about 30% below average. Rainfall is expected to remain below normal until mid-next year.
Sea surface temperatures are moving away from Thailand, affecting rainfall and storms entering the country. Thus, monitoring risk conditions is necessary, as there remains a moderate chance of storms forming over the sea.
Mr. Thanaroj explained that El Niño impacts Thailand by raising temperatures, which influences air pressure patterns. Cold air masses from China meet southwest winds from the Andaman Sea, causing heavy localized rains in short bursts, potentially triggering flash floods.
Areas unable to drain water quickly may experience flooding, flash floods, or water pooling awaiting drainage, but only for short periods. In contrast, the central plains and Bangkok will have scattered but less intense rainfall compared to mountainous areas.
Although rainfall is forecasted below average, current data shows increased inflows into reservoirs, such as Ratchaprapha Dam receiving over 30 million cubic meters and Vajiralongkorn Dam 25 million. This is a positive sign for water storage early in the rainy season to be used during the dry season. However, due to expected continued El Niño influence next year, water management efforts must be intensified.
Mr. Thanaroj concluded by advising the public to prepare for less rainfall and drought this year. Due to variable weather patterns, people should closely monitor conditions and not assume areas that rarely experience rain are safe from flooding.