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 snowmelt. 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
April 30, 2020
Snowmelt season is underway and the combination of above normal temperatures with below normal precipitation over the past month has led to accelerated melt rates and degrading snow drought conditions over large areas in the west with the maritime ranges being in the worst condition. Temperatures over the past seven days have been above normal over nearly the entire western US while early April warm anomalies were confined to mostly California, Oregon, and Washington with below normal temperatures elsewhere.
Northern California and Southern Oregon have seen some of the biggest decreases in percent of normal snow water equivalent (SWE) over the past month. From March 31 to April 26, the Klamath basin dropped from 75% to 50%, and Upper Sacramento basin from 96% to 66%. Low snowpack and precipitation, and above normal temperatures have been the primary drivers of drought for the region with widespread below normal streamflows and impacts to ranching and farming. Accumulated water year-to-date runoff is very low throughout much of western Oregon, and near historic lows in southwest Oregon. A two category downgrade of the US Drought Monitor has occurred in this region over the past month with D2 (severe drought) and D3 (extreme drought) currently depicted, resulting in 3 SW Oregon counties declaring state drought emergencies. The Humboldt Basin in Nevada is also in poor shape at 49% of normal SWE which will likely lead to drought impacts into the spring and summer as this basin provides surface and groundwater to major agricultural regions in northern Nevada.
For Colorado, winter-like storms have allowed the snowpack to continue to build in the northern half of the state. The Front Range had a particularly snowy April (37.3” of snowfall) that led to record seasonal snowfall at Boulder, Colorado with 152” since October 1 and broke the previous record of 134.7” set in 1909. Snow melt and below normal snowpack exists in the southern part of the state with the Upper San Juan basin, critical to Colorado River water supply, at 85% of normal.
For Alaska, snowpack continues to degrade in the Kenai Peninsula basin which is currently at 34% of normal SWE. Several waves of above normal temperatures combined with below normal precipitation have accelerated melt in this region. Turnagain Pass SNOTEL station, at 1880’ elevation, currently has the lowest SWE in the 37 years of record at 17.5” with the normal being 39.5”. All other stations in the Kenai Peninsula (with the exception Kenai Moose Pens) are at the 7th percentile SWE or lower based on SNOTEL stations.
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 26, 2020. Only stations with at least 20-years of data are included in the station averages. Shaded polygons show percent of median SWE for HUC-6 (hydrologic units) river basins. For an interactive version of this map please visit NRCS.
USDA Natural Resources Conservation Service (NRCS) water year accumulated snow water equivalent (SWE) (in.) (black) at Klamath Basin compared to 1981 - 2010 maximum (blue), median (green), and minimum (red). Graphics can be found at NRCS.
USDA Natural Resources Conservation Service (NRCS) water year accumulated snow water equivalent (SWE) (in.) (black) at Kenai Peninsula compared to 1981 - 2010 maximum (blue), median (green), and minimum (red). Graphics can be found at NRCS.