What is Snow Drought?
Snowpack typically acts as a natural reservoir, providing water throughout the drier summer months. Lack of snowpack storage, or a shift in timing of snowmelt from that reservoir, can be a challenge for drought planning. Few drought metrics include storage and release of snow water. Several years of low snowpack, especially across the western U.S., have led to many studies looking into the causes and impacts of reduced snow storage (see Resources) and the creation of a new definition of drought called Snow Drought.
Snow drought is defined as period of abnormally low snowpack for the time of year, reflecting either below-normal cold-season precipitation (dry snow drought) or a lack of snow accumulation despite near-normal precipitation (warm snow drought), caused by warm temperatures and precipitation falling as rain rather than snow or unusually early snowmelt. (AMS Glossary of Meteorology)
Snow-dominated regions face several challenges due to snow drought and its impacts:
- Summer Water Availability: Snow droughts reduce the amount of available water for spring and summer snow melt. This, in turn, reduces streamflow and soil moisture, which can have impacts on water storage, irrigation, fisheries, vegetation, municipal water supplies, and wildfire.
- Winter Water Management: Warmer winter storms lead to rain instead of snow at higher elevations in mountain regions that can create challenges for water management and flood mitigation strategies, particularly when dealing with extreme events.
- Outdoor Tourism and Recreation: Many local economies and industries rely on snowpack and river flows from snowmelt to support their outdoor industries such as skiing, rafting, and fishing.
- Ecosystems: Lack of snow can disrupt ecosystems over shorter and longer timescales.
Current Situation and Impacts in the West
June 27, 2019
This June 27 current situation update will be the last for the 2019 Water Year as snowpack and snowmelt are past peak values. Updates will resume in the 2020 Water Year (i.e., beginning October 2019). Access to the tools is available year round.
The transition into the summer season has brought continued warming and mountain snowmelt leaving a snowpack at only the highest elevation locations in the western U.S. and Alaska. South Central Alaska was one region that was impacted by snow drought throughout much of the 2019 water year. During peak snow water equivalent (SWE) accumulation season most locations never exceeded about 50-80% of long-term median. Within the western U.S., the Pacific Northwest was the primary region experiencing snow drought, specifically Washington, northern Idaho, and northwest Montana. April 1 SWE in this region ranged from around 75-85% of median, but a warm April-May with near-normal precipitation quickly depleted snowpack on the west slopes of the Washington Cascades. This lead to a rapid decline in snowpack to 10-30% of median SWE by June 1. Abundant moisture contributed to the development of a deep snowpack throughout the water year further south in the Sierra Nevada, Utah, and Colorado. This resulted in little concern of snow drought and associated impacts. As we move into summer, there are two primary snow drought-related impacts to consider in the Pacific Northwest. The first is reduced summer runoff and water supply. June-September runoff volume in the Washington Cascades is forecast to be about 40-70% of average with the greatest shortages expected in the Yakima River basin. The other concern is increased fire potential earlier in the fire season due to below-normal snowpack. A poor snow year, like 2019 was in Washington and northern Idaho, will lead to earlier melt out and more rapid drying of the soil moisture and vegetation. This element is one of several factors considered in the Predictive Services North American Significant Wildland Fire Potential Outlooks and latest outlook indicates above normal fire potential in July for western Oregon, western Washington, northern Idaho, and northwest Montana.
USDA Natural Resources Conservation Service (NRCS) snow water equivalent (SWE) values over the western U.S. (top) and Alaska (bottom) for June 23, 2019. White dots indicate zero SWE and no snow cover at the station location. For an interactive version of this map, including percent of period of station record median SWE, please visit NRCS.
USDA Natural Resources Conservation Service (NRCS) forecast percent of 1981-2010 average runoff volume for June-September 2019 at 50% exceedance probability over Oregon and Washington issued June 1, 2019. For an interactive version of this map, including percent of period of station record median SWE, please visit NRCS.