Uganda’s rainfall patterns are strongly influenced by large-scale atmospheric circulation phenomena that operate on timescales of years rather than seasons. El Niño and La Niña—the warm and cool phases of the El Niño–Southern Oscillation (ENSO) cycle—alter sea surface temperatures in the Pacific Ocean, which in turn shift atmospheric circulation patterns across East Africa with measurable effects on when rain falls, how intensely, and how reliably the seasonal calendar holds. For gorilla trekkers planning a Bwindi visit 12 to 18 months in advance, understanding the ENSO forecast and its implications for East African weather can improve the accuracy of seasonal planning.
The baseline: Uganda’s normal seasonal pattern
Uganda’s standard rainfall pattern operates through two rainy seasons: the long rains from March to May and the short rains from October to November, with dry periods in December-February and June-September. Bwindi is rarely truly dry—its position on the Albertine Rift escarpment captures moisture from both the Atlantic moisture conveyor (via the Congo basin) and the Indian Ocean monsoon system—but the June-September period is reliably drier, cooler, and less likely to produce the heavy daily rains that characterise April and May. This dry season is the foundation of the peak gorilla trekking calendar.
How El Niño affects Uganda
During El Niño events—characterised by anomalously warm water in the central and eastern Pacific—East Africa typically receives above-normal rainfall during the October to December period. In Uganda, this translates to prolonged short rains that extend further into November and December than usual, with heavier individual rain events. The 2015-16 El Niño, one of the strongest on record, produced severe flooding in parts of Uganda and Rwanda and significantly disrupted travel on the roads leading to Bwindi. Trail conditions during the short rains of El Niño years are more challenging than in neutral years, with increased mud, higher stream levels, and greater risk of landslides on steep road sections.
El Niño also tends to suppress the March-May long rains in East Africa, producing drier-than-normal conditions in the main wet season. For Bwindi trekkers, this means that the nominally rainy April-May period may actually offer better conditions in an El Niño year than usual—a counter-intuitive planning opportunity.
How La Niña affects Uganda
La Niña—the cool phase of ENSO—tends to produce the opposite effect in East Africa: enhanced long rains in March to May and potentially drier-than-normal conditions in October to December. La Niña years can produce flooding in the long rains season, with March and April seeing heavier-than-normal rainfall and stream levels in Bwindi that make some trail sections impassable. Conversely, the short rains may be shorter and less intense, and the dry season between June and September tends to be drier and longer than usual, producing excellent trekking conditions extending into October.
How to use ENSO forecasts in your planning
ENSO forecasts with reasonable accuracy are available 6-9 months in advance from meteorological agencies including NOAA, the UK Met Office, and the Australian Bureau of Meteorology. All publish regular ENSO outlooks on their websites that indicate whether El Niño, La Niña, or neutral conditions are expected in the coming seasons. For a trekker booking a permit 12 months in advance, checking the ENSO outlook as the booking date approaches—and adjusting destination sector or trekking date based on the forecast—can reduce the chance of encountering unusually bad conditions.
It is worth noting that ENSO influences probability rather than determining specific outcomes: an El Niño year does not guarantee flooding, and La Niña does not guarantee perfect dry weather. The interaction of ENSO signals with local topography, forest cover, and soil conditions creates significant variability within each season. Using the ENSO forecast as one input among several—rather than as a definitive predictor—provides a better basis for planning than either ignoring it or treating it as certain. Gorilla trekking is possible in all seasons and in all ENSO phases; the question is always one of expectation management and gear preparation rather than avoidance.
