Western US snow drought worsened after a December atmospheric river

An unusually warm atmospheric river in December 2025 brought heavy rain that melted much mountain snow, deepening a snow drought in parts of the western U.S.; some snow has since returned but key basins remained below historical norms in early January 2026.

Much of the western United States began 2026 experiencing a snow drought even after record precipitation from recent atmospheric rivers. Those storms delivered large amounts of moisture but fell as rain at many elevations because temperatures were unusually warm. Snow drought is measured as low snow water content relative to historical conditions, and a warm snow drought occurs when precipitation that would normally fall as snow falls instead as rain. The mountain snowpack is an important seasonal reservoir for downstream water uses, so changes in how and where precipitation falls matter for supply and flooding risks. Key points: - A snow drought means the snowpack contains less water than is normal for the season, even if total precipitation is not low. - A major atmospheric river in early December 2025 produced record precipitation in parts of the Pacific Northwest, with reports of heavy runoff, flooding and landslides in affected areas. - The storm mainly produced rain at lower elevations, with measurable snow largely limited to areas above about 9,000 feet, and warm conditions contributed to snowmelt during the event. - Rainfall and melting during the December event reduced snow water storage by about 50% in the Yakima River Basin over the course of the storm. - Additional snow since the storm has rebuilt some storage in the Yakima basin, but levels were still well below historical norms in early January 2026. - Researchers and water managers are working to improve forecasting of when storms will produce rain versus snow and to predict snow drought several weeks to months ahead. Summary: The December atmospheric river produced heavy rain and accelerated snowmelt, which reduced snow water storage in several western basins and contributed to runoff and localized flooding. More snow has fallen since the event and the main accumulation season runs through March, so some recovery is possible. Scientists and water resource managers are pursuing better forecasting to anticipate snow drought and the rain–snow boundary in individual storms. Undetermined at this time.