West Africa: Annual rainy season to occur across western regions through November

West Africa's annual rainy season will run through November, bringing an elevated threat of flooding, landslides, and mudslides across the region. During the rainy season, precipitation associated with the West African Monsoon starts to affect coastal areas in the Gulf of Guinea in March or April before migrating inland toward the Sahel in June. Monsoonal rains typically start to retreat southwards in September or October. Forecasts indicate above-average rainfall amounts forecast in parts of northern and western Burkina Faso, northern Cote d'Ivoire, the Gambia, Guinea-Bissau, northeastern and western Guinea, central and southern Mali, southern Mauritania, and Senegal, May-July. Normal to above-average rainfall amounts are expected in parts of northern Benin, southern Cote d'Ivoire, southwestern Ghana, southern Niger, and northern Nigeria, and normal to below-average amounts in parts of southeastern Nigeria. Average precipitation amounts are likely across the rest of the affected area May-July.

Heavy downpours, flooding, and landslides could affect business, transport, and utility services in many areas. Localized disruptions could persist well after hazardous events have concluded; floodwaters may take days to subside, and debris from mudslides may take weeks to be cleared. Rural communities and informal settlements often sustain the greatest damage and casualties during such events due to poor infrastructure. The risk of flooding is highest in low-lying communities near watercourses in recognized floodplains. Western Africa has several major rivers and lakes, including the Bandama, Gambia, Oti, Niger, Senegal, Volta, and White Volta rivers as well as Lake Chad and Lake Volta, that could flood during periods of intense rainfall. Flash floods often pose a threat to communities downstream of major rivers or dams in the event of structural 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.

Floodwaters and debris flows may render some bridges, rail networks, or roadways impassable, impacting overland travel in and around affected areas, especially in rural areas, where infrastructure is already of a poor standard. 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.

Significant downpours could trigger flight delays and cancellations at domestic and international airports in the region. Authorities may temporarily suspend port operations 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.

Localized business disruptions may occur; some businesses might not operate at full capacity because of flood damage to facilities, possible evacuations, and some employees' inability to reach work sites.

The threat of disease outbreaks is elevated during the rainy season. Backflow from drains that has 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.