DWR Continues to Improve Forecasting as Spring Heats up in California

By Karla Nemeth, DWR Director

For California water managers, the painfully sunny skies of March 2026 bear striking resemblance to March 2021.

In the spring of 2021, the snowpack was, like now, about half of average. Temperatures were breaking records. Then the news got worse. The snowmelt DWR forecasters expected to drain into reservoirs failed to appear. Unexpectedly, the mountain runoff disappeared into dry soils and a thirsty atmosphere. Caught off guard by this reality, DWR heeded this new climate signal.

The work done at DWR over the last five years to understand and track how snowpack translates into water supplymeans we know a great deal more about what to expect this dry, warm spring.

We do not control what, if any, rain and snow will fall in the next month and a half. But we will know a lot more about what water supply to anticipate, thanks to these forecasting improvements since 2021:

• DWR now factors six-to-10-day weather outlooks into its forecast models so that short-term events like extreme rain or heat are better reflected.
• DWR uses a new snow hydrology model (called iSnobal) in key watersheds that gives more explicit information on the changing physical parameters of the snowpack, including if and when the snow has become warm enough to melt and where.
• As funds allow, DWR hires[JI1] Airborne Snow Observatories, Inc.to fly over mountain snowpack to inventory water content and improve the accuracy of iSnobal.
• With UC Berkeley's Central Sierra Snow Lab near Donner Summit, DWR funds an experimental way to measure the temperature of the snowpack at various depths. Sometimes the layer of snow closer to the ground is warmer than the top, and snow is melting where we cannot see it. This, too, informs the iSnobal model.
• DWR has developed a soil moisture tool (see it here) that uses the limited number of soil-moisture sensors in the Sierra to estimate current conditions.
• DWR has developed a multi-source runoff forecast dashboard that includes runoff forecasts from federal, private, and academic partners such as the California-Nevada River Forecasting Center. The dashboard makes it easier to see snow-water equivalent information, giving DWR forecasters more information and perspective.

For 100 years, melting of the snowpack provided a key component of California water supply and typically occurred from April through July. But that historical pattern no longer holds true, and it makes water management a lot more complicated. It also reinforces the need to bolster overall state water supplies to compensate for a hotter, drier world. Thanks to Governor Newsom's 2022 Water Supply Strategy and newly-adopted Senate Bill 72, California is working to make up for a climate-driven loss of supplies.

The latest DWR runoff forecast, as of March 1, shows that snowmelt is well underway - more akin to what used to happen in May. Already 20 percent of the peak statewide snowpack is gone, and temperatures are stubbornly, unseasonably warm - even overnight in the Sierra Nevada mountains. Rivers are running higher than average now as the mountain snow melts. But big reservoirs like Shasta and Oroville cannot store much of that runoff, because this early in spring, the reservoirs must maintain enough space to protect people from flooding in case of a big late spring storm. California also lacks the right infrastructure to convey this early season runoff into underground aquifers.

The early loss of snowpack means that later in the summer, with limited cool water from melting snow, rivers could run lower than usual and create difficult conditions for native fish species including salmon.

All of this means that we must continue to restore floodplain and spawning habitatfor native fish species. It means water providers like the State Water Projectand Central Valley Project, which together supply 30 million Californians and 4 million acres of farmland, must be conservative about forecasting how much water supply they will be able to deliver to farms and cities this year.

It also means that local, state, and federal water agencies must keep investing in infrastructure that captures and conveys water to expanded and new reservoirs, such as San Luis and Sites, and to depleted aquifers throughout the state. To adequately meet the challenge of our hotter and drier climate, we must use water as efficiently as possible, recycle it where feasible, and capture and store it when the big storms arrive.