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
November 15, 2018:
We are now into mid-November, when climatologically, most mid-to-high elevation sites in the Western U.S. begin snowpack accumulation. After a record dry 2018 water year in some parts of the southern Rockies, New Mexico and Colorado are off to a great start for water year 2019. Many sites are at >200% of median snow water equivalent for the date. In contrast, the coastal states of California, Oregon, and Washington are all off to a slow start with SWE much below normal. Warm snow drought is already impacting Washington where water year-to-date precipitation is near or above normal in most locations. Southern Alaska is also beginning the water year in warm snow drought conditions. In interior central Alaska, both SWE and precipitation have been below normal. The southern two thirds of Oregon and California are currently experiencing dry snow drought conditions due to well below-normal precipitation this fall. To explore this more, compare the SWE maps below with station precipitation maps here.
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 November 12, 2018. 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.