Reno, Nev. (June 15, 2026) – While Reno families were celebrating the 1997 New Year, the Truckee River was surging into the city's downtown streets. A rainstorm was falling on the Sierra Nevada's deep snowpack, melting it rapidly and creating a hazardous situation for downstream communities. Rain-on-snow storms like this one have been challenging for emergency managers to forecast and prepare for due to limited understanding about how the snowpack responds to rain. A new tool directly addresses this problem by utilizing hourly soil moisture data from the SNOwpack TELemetry Network (SNOTEL) to provide insight into how the snowpack is responding to a storm in near real-time.
The tool is publicly available on DRI's website at https://snow.dri.edu/snowpackrunoff/ and is paired with a new publication validating its efficacy in Water Resources Research. The study applies the tool to the Upper Carson River watershed, which drains the eastern Sierra Nevada, finding that storms from 2006 to 2022 demonstrate its potential for capturing the timing of runoff. DRI scientist Anne Heggli led the effort with input from partners at the California-Nevada River Forecast Center, the National Weather Service, and the State of Nevada's Department of Transportation.
"We're trying to shave off uncertainty around how the snowpack responds to rain on snow," Heggli said. "Most days we don't have rain-on-snow events, but when we do, it can be millions, if not billions, of dollars in damage. We need to be prepared. This tool provides something tangible and useful for forecasters to use."
"The Snowpack Runoff Decision Support Tool is a great new resource," said Tim Bardsley, Senior Service Hydrologist at the National Weather Service and Heggli's collaborator. "It gives me near real-time information on how the snowpack is either mitigating or exacerbating runoff and flood risk leading into and during rain on snow events at a variety of elevations and snowpack conditions. This enhances our forecasting capabilities and strengthens our flood risk communication to our partners."
Beyond helping emergency managers better alert downstream communities to flooding events, tracking rapid runoff allows water managers to improve reservoir management. When large storms are expected, managers release water from upstream reservoirs to prevent flooding. If the runoff doesn't materialize, however, that water isn't available for use during the dry season. By monitoring the snowpack, and not just the subsequent streamflow, water managers have far more data much sooner. This allows them to optimize their decision making process.
"Reducing the uncertainty also allows us to optimize our reservoir operations to keep water up in the mountains, so that water managers are not releasing it down before it is necessary," Heggli said. "That's really where most of the tool's value is, because when really big atmospheric rivers come in, everyone just moves into flood operations, but the majority of our rain-on-snow events are more moderate in impact and there is a lot of uncertainty about how the snow will respond."
he tool, called the Snowpack Runoff Decision Support System, is possible because the Natural Resource Conservation Service added hourly monitoring to the SNOTEL stations beginning in the late 20th century. Although daily data is sufficient for many long-term water planning needs, the hourly data provides insight into the snowpack's runoff processes like never before. With as much as 75% of the West's water coming from mountain snowmelt, the breakthrough is critical for understanding every part of the water cycle. Cloud cover obscures satellite views during storms, making surface monitoring stations like SNOTEL the only data available on the condition of the snowpack.
"There's so much that we can continue to learn about these hydrological processes that have real economic and safety impacts on our communities," Heggli said. "There's a lot more that we can be doing to improve our decision making capacity and power, but we need to be able to access the data from these types of observational monitoring networks. It seems obvious to say that we can't learn from data if we don't have it, but with all the excitement about new tools like machine learning and artificial intelligence, we need to remember that the adage of 'garbage in, garbage out' gets amplified with these tools."
"A tool like this is important because it demonstrates the value of high quality, long-term observations from monitoring networks to optimize our systems to balance hazards and resources," said former DRI scientist Ben Hatchett, who co-authored the new study with Heggli and is now at Colorado State University. "This is especially true when co-developing tools with users. However, it also highlights the ongoing need to assess how new applications of data in frameworks like this ultimately influence the quality of decisions. Going forward, this points to the importance of applying social science to make further progress to ensure tools are not just usable but also operationally-relevant to meet the missions of varied user groups."
The Snowpack Runoff Decision Support Tool is currently available for the Sierra Nevada region of California and Nevada, including the Truckee and Carson River watersheds. A demo site also allows users to see how the tool makes runoff visible during significant regional rain-on-snow storms such as December 30, 2022 or the winters of 2005 and 2017. Heggli and her colleagues are currently partnering with East Bay Municipal Utility District (EBMUD) and PG&E to expand the tool's functionality to the Mokelumne River watershed. With additional funding support, the tool could potentially be expanded to all SNOTEL stations across the Western U.S.
Rain-on-snow storms are increasing in frequency, Heggli says, and are expected to increase between 15 and 25% by midcentury. Warming temperatures will continue to shift precipitation from snow to rain, increasing flood hazards for downstream communities. Beyond the Western U.S., these types of storms are also expected to increase in regions like Norway and the Alps.
"As the atmospheric temperature warms, there's more water in the atmosphere, which can cause more extreme events, but we haven't yet warmed enough that snow is impossible," Heggli said. "You could say that we're approaching a period of peak rain on snow."
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More information: The full study, The Application of a Snowpack Runoff Decision Support system for Rain-on-Snow Events, is available from Water Resources Research at https://
The Snowpack Runoff Decision Support Tool is available at https://snow.dri.edu/snowpackrunoff/ and you can learn more about DRI's snow research at https://www.dri.edu/snow/
To learn more about rain-on-snow events and Anne Heggli's research, read DRI's blog story: Understanding Rain-on-Snow Events with Anne Heggli
Study authors include: Anne Heggli (DRI), Ben Hatchett (Colorado State University), Tim Bardsley (NWS Reno), Mike Imgarten (CA-NV River Forecast Center), and Pete Fickenscher (CA-NV River Forecast Center)
DRI's Snowpack Runoff Decision Support Tool was made possible with the help of Andy Joros, Research Computing Engineer.
About DRI
We are Nevada's non-profit research institute, founded in 1959 to empower experts to focus on science that matters. We work with communities across the state — and the world — to address their most pressing scientific questions. We're proud that our scientists continuously produce solutions that better human and environmental health.
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