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
January 9, 2020
The large scale spatial pattern of snow water equivalent (SWE) anomalies in the Lower 48 has not changed much since our last update in early December: the Pacific Northwest generally remains below normal with a gradient to above normal as you move southeast into the Four Corners. However, the magnitude of these anomalies has changed significantly due a series of storms bringing above normal precipitation (for the past three weeks) to the Washington Cascades and a drying trend to the Sierra Nevada and Intermountain West. The core of the worst snow drought conditions has now shifted from the Washington Cascades to the central and northern Oregon Cascades. For HUC-6 Basins the Willamette Basin in Oregon currently has the lowest snowpack at 37% of median SWE while Washington Basins has improved to 55-75% of median. As of January 6 there are now no stations reporting record low SWE and only four stations in the Pacific Northwest reporting 2nd lowest SWE on record with one in Oregon, two in Wasington, and one in Montana.
Despite large improvements in precipitation deficits in the Washington Cascades, SWE gains were limited at lower elevations due to warm temperatures and rain instead of snow for much of December. Alpine Meadows SNOTEL, on the west slope of the Washington Cascades at 3500’ elevation, recorded 34.1” of precipitation between December 9 and January with only 11.5” of SWE gains. The most recent storms over the past week have brought colder temperatures and more snow accumulation at lower elevations compared to mid-December storms.
The poleward shift in the storm track has led to a drying trend south of the Cascades. Major storms in the Sierra Nevada have been absent since mid-December and many locations have fallen to near-to-slightly below normal SWE for this time of year. Small SWE losses have even occurred at some locations in the past week. At the Central Sierra Snow Lab, near Donner Pass California, SWE has declined from 13” on December to 12” on January 7 with 87% of median SWE. Overall, the Sierra is still in good shape with the Walker River Basin being the only HUC-6 below normal at 90% of median SWE.
Snowpack in south-central Alaska remain below normal with Kenai Peninsula and Prince William Sound HUC-6 basins at 56% and 50% of median SWE, respectively. In southeast Alaska there is only one station with long enough records to computes normals: Long Lake currently sits at 91% of median SWE. The few stations in the interior of Alaska currently reporting data all indicate above normal snowpack.
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 January 5, 2020. Only stations with at least 20-years of data are included in the station averages. For an interactive version of this map please visit NRCS
USDA Natural Resources Conservation Service (NRCS) percent of 1981-2010 average precipitation over the western U.S. for the period December 16, 2019-January 5, 2020. Only stations with at least 20-years of data are included in the station averages. For an interactive version of this map please visit NRCS.
University of Idaho’s gridded meteorological data (gridMET) mean temperature difference from the 1981-2010 average for the period December 13, 2019-January 3, 2020. For an interactive version of this map please visit Climate Engine.