Spotlight |
Forecasting the atmospheric rivers
Advanced hydrological forecasting helped dam operators across the Pacific Northwest balance flood risk and reservoir operations during December 2025’s unprecedented atmospheric river events.
GEOS visualisation showing atmospheric river moisture transport and precipitation accumulation across the Pacific Northwest, December 2025. NASA
IN DECEMBER 2025, a series of atmospheric river (AR) storms brought record flooding to the Pacific Northwest (PNW), testing the resilience of hydropower operators and regional water managers. After months of severe drought, hydrologists and dam operators were suddenly required to shift from managing critically low reservoirs to responding to intense inflows within a matter of days. This rapid transition placed significant operational pressure on teams, as decisions that would normally unfold over weeks had to be made in near real time. During the events, HydroForecast provided
extended lead-time predictions that helped operators prepare for rapidly changing hydrological conditions. At several customer sites, the company identified peak flows up to 10 days in advance – four days earlier than forecasts issued by the Northwest River Forecast Center (NWRFC). This additional lead time created a critical planning window, allowing operators to assess risks, review contingency plans and begin coordinating responses ahead of the first major inflows. For utilities including Tacoma Power, Seattle City Light and Evolugen, this earlier warning proved critical in supporting operational decision-
10 | April 2026 |
www.waterpowermagazine.com
making. By combining forecast signals with expert interpretation from HydroForecast’s hydrology team, operators were able to communicate release schedules to emergency response partners and flood control authorities such as the US Army Corps of Engineers (USACE). The result was improved coordination between agencies at a time when operational clarity was essential, helping to reduce uncertainty and ensure that actions taken upstream were aligned with downstream preparedness efforts.
Balancing flood control and reservoir
operations The first wave of atmospheric rivers provided relief from drought conditions as reservoirs began to refill. However, the speed and intensity of the inflows required operators to carefully manage available storage while preparing for additional storm systems. What began as a welcome recovery in water levels rapidly evolved into a complex flood management challenge. Using extended-range forecasts, operators were
able to absorb the initial surge while maintaining capacity for subsequent waves of rainfall. With up to 10 days of advance visibility, reservoir managers
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