NOAA Update

Alan Haynes, Hydrologist in Charge, California Nevada River Forecast Center

Driest 3-YR Cumulative Runoff Expected for Lake Shasta

Water Year 2022 started off promising with a couple of wet events in the early part of the water year, followed by three large Atmospheric River events in December. However, the weather turned dry in January and stayed dry until April; perhaps the driest Jan-Feb-Mar on record for the Sierra Nevada and Central Valley. Consequently, water-year runoff forecasts for the combined Central Valley River systems reached a peak in late December, around 12,300 kaf (full natural flow), but then dropped to 3,800 kaf by the end of March. Some wet weather in the first half of April over Northern CA helped bring forecast runoff up to 5,300 kaf, which is about 60% of the mean. However, this forecast runoff follows two consecutive dry years, making the expected 3-yr cumulative runoff about 48% of the mean. One of the hardest hit reservoirs in this latest dry period is the state’s largest reservoir, Lake Shasta, with a storage capacity of around 4.5 million acre-ft. The cumulative 3-yr runoff into Lake Shasta is expected to be the lowest on record, around 8700 kaf or about 50% of the mean (figure 1). The previous record low in the 3-yr runoff was 10,300 kaf, in the 1990-1992 period. The cumulative impacts of this low runoff can be seen in the current storage at Lake Shasta (figure 2), which is around 1800 kaf as of late April. This is near 50% of the historical average, or about 40% of capacity.

Figure 1. Top ten lowest 3-yr runoff into Lake Shasta. The 3-yr period ending this water year is projected to be nearly a half million-acre ft lower than the previous lowest runoff in the 1990-1992 period. Note that nine of the top ten lowest runoff volumes have occurred since 1

Figure 2. Current storage information for Lake Shasta as of April 26th, 2022.

Expanded Atmospheric Model Input Into HEFS

The California Nevada River Forecast Center (CNRFC) runs its hydrologic model in two modes: 1) deterministic mode - single-value forecasts go out five days. 2) probabilistic mode - multiple instances of the hydrologic model are run, with the same initial conditions, but with varying meteorological inputs. These forecasts are produced using the Hydrologic Ensemble Forecast Service (HEFS) and they go out to a year. Until recently, weather forecasts were used in HEFS for the first two weeks, with climatology beyond that. Recently, the CNRFC determined that there is some skill in the atmospheric model in weeks 3-4. Thus, atmospheric model forecasts are now used out through week four in the CNRFC’s HEFS forecasts. Users shouldn’t expect to see much change, but the CNRFC is always trying to improve its products and services as opportunities arise.

Staffing

The California Nevada River Forecast Center hired a new meteorologist who started in the fall of 2021, Anna Schneider. Anna Schneider was previously a Meteorologist at the NWS San Francisco/Monterey Forecast Office since August 2016. Anna attended Penn State University for both undergraduate and graduate degrees in Meteorology. Currently, the CNRFC has two hydrologist vacancies that it is currently trying to fill. Ideally, these positions will be filled, and the new forecasters trained before the next wet season. 19