Anne Heggli was knee-deep in a snow pit on a stormy Sierra Nevada slope when the first drops of rain began to fall, her gloves soaked as she measured how quickly the snowpack responded. That hands-on research, part of her Ph.D. work, laid the foundation for a breakthrough tool now helping communities across the Western U.S. prepare for one of the region’s most dangerous weather phenomena: rain-on-snow events. These storms, where warm rain falls on deep snow, can trigger sudden, devastating floods—like the one that sent the Truckee River surging through downtown Reno during New Year’s weekend in 1997. For decades, forecasters struggled to predict how the snowpack would react, leaving emergency managers guessing. Now, thanks to Heggli’s Snowpack Runoff Decision Support Tool, they have real-time insight.
The tool leverages hourly data from the SNOwpack TELemetry (SNOTEL) network—longtime monitoring stations managed by the Natural Resources Conservation Service—offering a granular view of snowpack behavior during storms. Previously, most models relied on daily data, which wasn’t enough to capture the rapid changes during rain-on-snow events. By analyzing soil moisture and snowpack response at multiple elevations, the tool helps forecasters determine whether the snow is absorbing rain or accelerating runoff. It’s already been tested across 16 years of storms in the Upper Carson River watershed, proving its ability to pinpoint runoff timing with unprecedented accuracy.
Heggli, now a scientist at the Desert Research Institute (DRI), developed the tool in collaboration with the California-Nevada River Forecast Center, the National Weather Service, and the Nevada Department of Transportation. "We're trying to shave off uncertainty around how the snowpack responds to rain on snow," she said. "Most days we don't have rain-on-snow events, but when we do, there can be millions, if not billions, of dollars in damage. We need to be prepared."
For water managers, the implications go beyond flood warnings. Reservoir operators often release water ahead of big storms to prevent overflow, but if the snowpack absorbs the rain instead of releasing it as runoff, that released water is lost—potentially needed later in the dry season. With this tool, managers can delay unnecessary releases, keeping more water stored in mountain reservoirs until it’s truly needed. "Reducing the uncertainty also allows us to optimize our reservoir operations to keep water up in the mountains," Heggli explained.
The tool’s success underscores a broader truth: high-quality, long-term environmental data is the backbone of climate resilience. As atmospheric rivers grow more intense with climate change, tools like this one—grounded in decades of observation—will be essential for balancing flood safety with water security. "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. "We need to remember that the adage of 'garbage in, garbage out' gets amplified with these tools."
Now publicly available on DRI’s website and validated in the journal Water Resources Research, the Snowpack Runoff Decision Support Tool is more than a scientific achievement—it’s a practical shield for communities and a smarter lever for water managers navigating an increasingly unpredictable climate.