Multi-Year Forecasts of Colorado River Water Supply

Perspective

Ocean precursor forecasts suggest an improvement in water shortage conditions for the next several years. Although our forecast failed to capture the observed wet year of 2023, uncertainty remains regarding whether the Intermountain West is heading into a wet decade. A strong El Niño event occurred last winter, and multi-year La Niña events are expected in the upcoming winters of 2024/25 and 2025/26. Our recent study indicates that consecutive La Niña events tend to favor dry conditions in the Intermountain West, which may lead to a return of drought. Nevertheless, Utah's snowpack was slightly above normal this winter, and severe drought threats have not been detected in the ocean precursor forecasts. (updated Jul 30, 2024)

Figure 1: Forecasts of Ocean-Induced Colorado River Water Supply. The blue and red lines represent the observed water supply provided by the Bureau of Reclamation and the forecasts based on ocean precursors, respectively. The thin and thick lines correspond to annual and three-year averages based on the water year. The forecast is derived by averaging three forecasted water supply products based on three ocean datasets (see Figure 2 below).

The next forecast will be available in the 2025 Summer.

Technical Contents

Figure 2: Timeseries show the Colorado River water supply (blue) and its forecasts (orange). The Colorado River water supply corresponds to the water year natural streamflows at Lees Ferry, AZ (Bureau of Reclamation). Forecasts rely on two ocean precursors: sea surface temperature anomalies in the North Pacific 29 months ago and the southern tropical Atlantic 44 months ago. We calculated the forecasts based on three different sources of the observed sea surface temperature datasets: COBS SST, OI SST, and ERSSTv5. These forecasts indicate the ocean-induced multi-year drought threats (Chikamoto et al., 2020), explaining about 25% of the total variance. The short atmospheric drought is unpredictable in our model. Please check the seasonal predictions if you are interested in a medium-range forecast.

Ocean Precursors

1964 1967 1977 1991 2001 2002 2003 2020 2021 2022 2023 2024 2025 2026

Figure 3: Our multi-year forecasts of the Colorado River water supply result from combining two ocean precursors. The likelihood of a severe natural streamflow reduction in the Colorado River increases when sea surface temperatures are warmer than usual in the southern tropical Atlantic 44 months ago and the North Pacific 29 months ago. Historically, our ocean drought indicator (represented by the orange lines above) has detected eight events of natural streamflow reduction: in the water years of 1964, 1967, 1977, 1991, 2001, 2002, 2003, and 2020. The graph below shows sea surface temperature anomalies at 44 months (left) and 29 months ago (right) for specific years (upper panels) and for all years composite (lower panels). You can check these ocean precursors by selecting the year from the toggle. Ocean precursors for the current and next water years are also available.