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The month of October 2025 was marked by rainfall above the historical average in Rio Branco, according to data from the National Center for Natural Disaster Monitoring and Alerts (Cemaden) and Municipal Civil Defense. The expected average for the period is 150 millimeters, but the observed volume reached 204.6 mm, representing an increase of 36.5%.
This was the wettest October in the last five years, with the exception of 2022. The increase in rainfall directly impacted the level of the Acre River, which reached 2.77 meters on October 31st — the fifth highest level recorded in the last eleven years. Only 2017, 2018, 2019 and 2024 presented higher levels.
Above-average rainfall in October raises the Acre River and triggers a flood alert in early 2026/Photo: ContilNet
According to technicians, since the second half of October, the region has been under the influence of the La Niña phenomenon, characterized by the cooling of the surface waters of the Equatorial Pacific Ocean. The phenomenon tends to cause an increase in rainfall in several regions, including Acre.
Graph shows variation in the level of the Acre River on October 31st of each year, between 2015 and 2025/Photo: Reproduction
The forecast is that La Niña will intensify throughout November, increasing the probability of precipitation above the historical average, which is 206.1 mm for the month. As a result, the tendency is for the level of the Acre River basin to begin to rise at the end of November, potentially reaching the overflow level in the first months of 2026.
Still in November, the monitoring and civil defense bodies must hold a “pre-flood” meeting, in which a detailed prognosis will be presented on the hydrological behavior in the transition between 2025 and 2026, with a special focus on the first quarter of next year.
Full note:
The month of October was the first this year, after April, to record above-average rainfall. The historical average is 150 mm, and the observed volume was 204.6 mm, representing an increase of 36.5% compared to expected.
Thus, it was the wettest October in the last five years, with the exception of 2022 (see the graph).
As for the river level, 2025 presents the fifth best index in the last eleven years. Only the years 2017, 2018, 2019 and 2024 recorded higher levels, while 2015, 2016, 2020, 2021, 2022 and 2023 presented lower levels (see the graph).
Since the second half of October, we have been under the influence of the La Niña phenomenon — characterized by the cooling of surface waters in the Equatorial Pacific Ocean, which impacts the rainfall regime in several regions of the planet. The expectation is that the phenomenon will intensify throughout November, increasing the probability of precipitation above the historical average for the period (November), which is 206.1 mm.
Therefore, the level of the Acre River basin should begin to rise at the end of November, with a tendency to increase in the following months. If the scenario is confirmed, there is a possibility that the overflow quota will be reached in the first months of 2026.
During this month, the relief and response bodies, together with the climate monitoring and forecasting bodies, are expected to hold a “pre-flood” meeting, at which time a more detailed prognosis will be drawn up on hydrological behavior in the transition from 2025 to 2026, especially focusing on the first quarter of 2026.
