The Napa County ALERT Flood Warning System, often called the “OneRain” system (napa.onerain.com), is a network of hydrologic monitoring stations deployed around the Napa Valley. The stations report rainfall and water levels in real time and warn local emergency managers of intense downpours and quickly rising streams that could result in flooding. In addition, the data collected provide an accurate and detailed look at the timing and distribution of rainfall throughout the valley.
Hydrologic data are organized according to the “water year” which begins on October 1 and goes through September 30. As of May 1, 2026, we are seven months into Water Year 2025-26 and based on long-term averages we have typically received 94% of our annual rainfall by this point. Annual rainfall totals for Water Year 2025-26 to date range from 22.7 inches in the City of Napa to 39.7 inches at the top of Mt. St. Helena. These totals represent approximately 80% of our mean annual precipitation, indicating that this has been a below average water year.
Monthly rainfall totals closely tracked the long-term averages with a slightly wet November and December offsetting a slightly dry January and February. A nearly completely dry March dropped the annual total well below the average and a slightly wet April was unable to make up the difference. Although annual rainfall has so far been below average, it does not classify as a “dry” water year and remains just within the “average” water year classification. This would be the third average year in a row since the “wet” Water Year 2022-23.
Paul Blank
Program Manager, Environmental Monitoring