India, Sri Lanka: Increased threat of flooding and landslides during northeast monsoon period in southern India and Sri Lanka through December

There is an increased likelihood of flooding, landslides, mudflows, and related disruptions across portions of Andhra Pradesh, Karnataka, Kerala, and Tamil Nadu states, and Puducherry Union Territory in southern India, as well as across Sri Lanka during the northeast monsoon period through at least December.

The monsoon season produces regular rainfall activity; the heaviest precipitation tends to fall in coastal regions in southeastern India. Although the northeast monsoon does not usually commence in Sri Lanka until December, rounds of heavy rainfall are also typical during an inter-monsoon season that takes place in November. Flash, areal, and urban flooding are possible in the affected area over the coming weeks and months.

Tropical systems, including depressions and cyclones, pose an additional concern during the northeast monsoon period through December. These systems occasionally form in the Bay of Bengal and can track northwestward over Sri Lanka and the Indian subcontinent, bringing destructive winds and rain. Tropical systems usually prolong and/or aggravate disruptions already caused by monsoon rainfall.

Heavy downpours, flooding, and landslides could affect business, transport, and utility services throughout the 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 their poor infrastructure.

The potential for flooding is highest in low-lying communities near watercourses in recognized floodplains. Rivers such as the Bharathappuzha, Godavari, Kaveri, and Periyar in India and Kalu, Kelani, Mahaweli, Malvathu, Walawe, and Yan Oya in Sri Lanka, 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, inadequate, or nonexistent drainage systems. Flooding is a perennial problem in Chennai, Bengaluru, Mysuru, Thiruvananthapuram, Visakhapatnam, and several other major cities in southern India. Storm surges may trigger flooding in coastal areas. Rain-induced mudflows and landslides pose a significant threat in elevated terrain such as the Central Highlands in Sri Lanka; structures built on unstable, steep slopes face the greatest threat from this hazard.

In addition to posing a direct threat to personal health and safety, flood and landslide events can trigger localized business, transport, and utility disruptions in southern India and Sri Lanka. 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.

Severe weather could also trigger flight delays and cancellations at domestic and international airports such as Chennai International Airport (MAA), Tiruchirappalli International Airport (TRZ), and Trivandrum International Airport (TRV) over the coming months. Authorities may temporarily suspend port operations or close beach fronts if strong winds trigger hazardous sea conditions. Flooding could block regional rail lines; freight and passenger train delays and cancellations are likely in areas that see heavy rainfall and potential track inundation.

Residual disruptions could persist well after weather conditions improve, particularly where cleanup or repair operations are required. Business closures and unscheduled employee absences during the northeast monsoon season are likely in flood-prone areas; officials may also declare holidays.

The threat of disease outbreaks cannot be discounted during flooding episodes. Backflow from drains often mixes with floodwaters and can become trapped in open areas when inundations recede. These stagnant pools become a breeding ground for mosquitoes and bacteria, increasing the incidence of insect- and waterborne diseases. The threat of such disease outbreaks is elevated in the low-income areas of major urban centers due to the presence of incomplete or open sewer lines. Exposure to contaminated water from inundated industrial sites, sewer systems, and septic tanks also poses a significant health threat.