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
February 7, 2019:
Mid-winter snowpack is near or above normal in a belt across the middle latitudes of the western US including the Sierra Nevada, Great Basin, Utah, and Colorado. After a good start to the winter, storms with warmer temperatures and higher snow levels have led to a rapid decline of snowpack in the mountains of central Arizona and southwest New Mexico creating warm snow drought conditions. For example, the Salt River Basin in central Arizona currently sits at 71% of median snow water equivalent despite accumulated water year precipitation being at 95% of median. The other area of snow drought in the lower 48 includes western Washington and Oregon. These areas are also in a warm snow drought although precipitation has also been below normal. In Oregon’s Willamette Basin, conditions are currently at 56% of median snow water equivalent and accumulated water year precipitation is at 79% of median. Throughout northwest Wyoming, Idaho, and western Montana conditions are mixed with pockets of above and below normal snowpack present. Nearly all stations south of the Arctic Circle in Alaska currently have below normal snowpack. This has been driven by a lack of precipitation in the Interior and southeast Alaska while the southern basins including Prince William Sound, Kenai Peninsula, Knik Arm, and Susitna River all have above normal water year precipitation indicating warm snow drought conditions.
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 February 3, 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.
USDA Natural Resources Conservation Service (NRCS) SNOTEL stations in the Oregon (top) and Washington (bottom) Cascades on February 4, 2019. Percent of 1981-2010 median snow water equivalent (SWE) is plotted on the horizontal axis against station elevation on the vertical axis. Dots are colored by accumulated water year precipitation percent of 1981-2010 average. Warm snow drought is easy to identify by the strong increase in SWE with elevation driven by warm temperatures and rain at lower elevations for much of the winter. In the Washington Cascades (bottom) five stations, all below 1200 m elevation, currently are below 40% of median SWE with 80-120% of average water precipitation.