Asia: Southwest monsoon season forecast to bring flooding and landslides across the region through September

Event

The 2023 southwest monsoon period will continue to bring an elevated potential for flooding and landslides/mudslides across South Asia and northern Southeast Asia through Sept. 30. The affected area includes India, Pakistan, Sri Lanka, Myanmar, Bhutan, Bangladesh, Nepal, Cambodia, Laos, Thailand, the Philippines, and Vietnam. Monsoon rains typically begin to impact the southern portions of the region in late May and gradually spread to the north and west, encompassing the entire region by early July. The monsoon does occasionally deviate from this range due to complex interactions with the atmosphere, regional oceans/seas, and landmasses. Authorities have confirmed that the monsoon started around a week earlier this year. Forecast models indicate below-average to average rainfall in most of South Asia and southern Southeast Asia for the 2023 monsoon season.

Hazardous Conditions
Heavy downpours, flooding, and landslides could affect business, transport, and utility services across South Asia and northern Southeast Asia throughout the 2023 monsoon season. Localized disruptions could persist well after hazardous events have concluded; it could take days or weeks for floodwaters to subside and for authorities to clear landslide debris. Rural communities and informal settlements often suffer the greatest damage and casualties during such events due to poor infrastructure.

This potential for flooding is highest in low-lying communities near watercourses in recognized floodplains. Rivers such as the Indus, Ganges (Ganga), Padma, Meghna, Jhelum, Chenab, Ravi, Sutlej, Bagmati, Gandaki, Koshi (Kosi), Amo Chu, Manas, Sankosh, Raidak, Chindwin, Ayeyarwady (Irrawaddy), Sittaung, Thanlwin (Salween), Yangon, Brahmaputra, Attanagalu Oya, Kalu Ganga, Mahaweli, Nilwala Ganga, Yan Oya, Nan, Chao Phraya, Red, Cagayan, and Mekong, as well as their tributaries, regularly overflow their banks during periods of intense rainfall. Flash floods, which often occur with little-to-no warning, can pose a threat to communities downstream of major rivers or dams in the event of infrastructural failure. Urban flooding is possible in areas with poorly maintained or inadequate drainage systems, while storm surges may trigger flooding in coastal areas. Landslides and mudslides are possible in hilly and mountainous areas, including the Western Ghats and Konkan Hills (India), Khyber Pakhtunkhwa and Gilgit-Baltistan (Pakistan), eastern Himalayas (Bhutan), Chittagong Hill Tracts and Rang Tlang mountain range (Bangladesh), Chin Hills and Pegu Mountains (Myanmar), and the Central Highlands (Sri Lanka).

Tropical systems, including tropical depressions, storms, and cyclones, pose an additional threat during the southwest monsoon period. Storms associated with the season occasionally form in the Arabian Sea, Bay of Bengal, and northern Indian Ocean; these weather systems sometimes track over South Asia and can become very destructive. Tropical systems usually prolong or aggravate disruptions already caused by monsoon rainfall.

Transport
Floodwaters and debris flows may render some bridges, rail networks, or roadways impassable, impacting overland travel in and around affected areas. Ponding on road surfaces could cause hazardous driving conditions on regional highways. Authorities could temporarily close some low-lying routes that become inundated by floodwaters. In the event of flooding, localized business disruptions may occur in low-lying areas; some businesses might not operate at full capacity because of flood damage to facilities, possible evacuations, and some employees' inability to reach work sites.

Health
The threat of disease outbreaks cannot be discounted during the monsoon season. Backflow from drains mixed with floodwaters can become trapped in open areas when inundations recede. These stagnant pools often become a breeding ground for mosquitoes and bacteria, increasing the incidence of insect- and waterborne diseases. Exposure to contaminated water from inundated industrial sites, sewer systems, and septic tanks also poses a significant health threat.

Advice

Monitor local media for weather updates and related advisories. Confirm all transport reservations and business appointments before travel. Make allowances for localized travel delays and potential supply chain disruptions where flooding has been forecast. Do not drive on flooded roads. Charge battery-powered devices in the case of prolonged electricity outages.

Resources

India Meteorological Department
Bangladesh Meteorological Department
Department of Meteorology and Hydrology, Myanmar
Department of Hydrology and Meteorology, Nepal
Pakistan Meteorological Department
Department of Meteorology, Sri Lanka
Department of Meteorology, Cambodia
Thai Meteorological Department (Thai)
Hydrometeorological Service, Vietnam (Vietnamese)
Philippine Atmospheric, Geophysical and Astronomical Services Administration