Rwanda’s weather forecasts are designed to guide decisions, not to predict the atmosphere with absolute certainty, meteorological experts have said, as the country faces a September-December season expected to bring above-normal rainfall.
The Rwanda Meteorology Agency (Meteo Rwanda) has forecast a high probability of above-normal rainfall across the country during the September-December 2026 season, with cumulative rainfall expected to range between 300 and 1,000 millimetres, compared with a long-term average of 279 to 962 millimetres.
But understanding what such a forecast actually means is as important as receiving it.
Alphonsine Musanganire, an Environmental and Social Risk Management Specialist at Meteo Rwanda, explained that meteorological information comes with different levels of certainty depending on the forecast period and the information available.
Forecasts can range from nowcasting, covering the next one to two hours, to very short-range forecasts of up to 12 hours, short-range forecasts covering 12 to 72 hours and medium-range forecasts extending from three to 10 days. Longer-range forecasts cover periods beyond that, while climate projections look further into the future.
“Forecasts can be presented either deterministically, where a particular expected condition is described, or probabilistically, where the likelihood of an event occurring is expressed,” Musanganire said.
For example, a forecast may indicate that Musanze could receive between 20 and 40 millimetres of rain the following day, alongside a 70 percent probability of rainfall.
For context, 20 millimetres of rainfall is equivalent to 20 litres of water falling on an area of one square metre.
The probability does not mean that meteorologists can guarantee that rain will occur. Rather, it communicates the likelihood of the defined event based on available observations, forecasting methods and models.
That distinction becomes particularly important when interpreting seasonal forecasts.
Meteo Rwanda’s SOND 2026 outlook, released on August 21, says rainfall during the season is expected to be above the long-term mean across the country. The agency expects the season to be influenced by El Niño, warming in the western Indian Ocean associated with a positive Indian Ocean Dipole, and the movement of the Inter-Tropical Convergence Zone.
The forecast places expected seasonal rainfall between 800 and 1,000 millimetres in Nyamasheke, western parts of Rutsiro and Karongi, western and southern parts of Nyamagabe and Nyaruguru, and eastern parts of Rusizi. Other parts of the country are expected to receive between 300 and 800 millimetres.
Meteo Rwanda says the expected rainfall pattern could resemble those observed in 1997 and 2023, although analogue years are used as guidance and do not mean the coming season will repeat past events exactly.
Why uncertainty matters
“Uncertainty is an inherent part of weather forecasting because the atmosphere is highly complex and cannot be observed everywhere at every altitude and moment,” Musanganire said.
Meteo Rwanda operates about 350 weather stations across the country, with an average distance of 10 kilometres between stations, providing observations that feed into weather forecasting.
Weather models are also approximations of atmospheric processes, while small differences in initial atmospheric conditions can produce different outcomes later. The further into the future a forecast looks, the greater the uncertainty generally becomes.
“Local weather can also change rapidly. Thunderstorms, for example, can develop or shift location over relatively short periods,” Musanganire added.
This is why a seasonal forecast should not be interpreted as a day-by-day prediction.
The World Meteorological Organization similarly notes that climate forecasts are probabilistic in nature and that forecast accuracy generally decreases with longer lead times. It recommends using seasonal information alongside shorter-term forecasts and other climate-monitoring products.
For Rwanda, that distinction is particularly relevant because the rainy season is expected to bring increased risks from heavy rainfall, strong winds, lightning, hailstorms, runoff and soil erosion.
Meteo Rwanda has advised institutions, businesses, farmers and the public to use the seasonal outlook to support planning while also consulting complementary forecasts and weather warnings as the season progresses.
The Ministry in Charge of Emergency Management (MINEMA) has already mobilised communities ahead of the season, encouraging measures such as clearing drainage channels, securing roofs, strengthening erosion-control structures and relocating people living in high-risk areas.
MINEMA has also identified disaster-risk hotspots where floods and landslides pose significant threats. A recent national assessment identified 326 hotspots, including 134 classified as high or very high risk.
