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
May 2, 2019:
Snowmelt season is now underway across the Western US. Above normal temperatures for the month of April have accelerated this process for many locations. In western Oregon rapid melt has led to below snow water equivalent (SWE) in places that were above normal SWE just a month ago on April 1. The Willamette Basin changed from 104% of normal SWE on April 1 to 78% of normal on April 29 and similarly, the Deschutes Basin changed from 104% to 77% of normal SWE. The Washington Cascades are in the worst snow drought conditions overall from a regional perspective with complete melt out having already occured at some lower elevation locations about 2-3 weeks ahead of climatological melt out dates. The Sierra Nevada currently boasts the largest SWE values in the Western US and sits at 150% to >200% of normal. At higher elevations in the Sierra Nevada, current SWE values are still between 40-60 inches. At lower elevations in the Sierra Nevada, abundant snowpack is still present in locations that have typically melted out by now like Tahoe City, California where the current SWE is 9.1 inches and the climatological melt out date is April 24. Southern coastal Alaska remains in snow drought with a number of stations less than 50% of normal SWE and two weeks ahead of normal melt out as demonstrated at Moraine which has no snow remaining, but has a typical melt out date of May 13.
USDA Natural Resources Conservation Service (NRCS) percent of 1981-2010 median snow water equivalent (SWE) over the western U.S. (top) and Alaska (bottom) for April 28, 2019. Only stations with at least 20-years of data are included in the station averages. For an interactive version of this map, including percent of period of station record median SWE, please visit NRCS.