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
February 6, 2020
Dry conditions relative to climatology have persisted over the past month in the Sierra Nevada, California and Nevada, leading to the lowest snow water equivalent (SWE) percent of median at the HUC-6 river basin scale in the lower 48. All of the Sierra Nevada HUC-6 basins are currently below 80% of median SWE, with the Walker River basin in the eastern Sierra Nevada the lowest at 61% of median. Small storms throughout January allowed for minor snow accumulations in the region, but this was not enough to keep up with climatology as January is one of the wettest months of the year. While snowpack is below normal for the Sierra Nevada, conditions are much improved compared to early February numbers in other recent snow drought years such as 2015 and 2018, when record, or near-record, low SWE was measured for the same time period.
The biggest change in snow drought conditions since early January has been in the Washington Cascades. A number of SNOTEL stations in the region recorded the wettest January on record that led to substantial SWE gains and rapid changes from near record low to slightly above normal snowpack. However, the percent of median SWE is not distributed evenly across elevation with lower elevation stations still only at 50-80% of median. Storms during early January and again during the first week of February have been accompanied by warm temperatures and high freezing levels. This has led to decreasing SWE in some cases at lower elevations.
Other areas of concern in the lower 48 include the Sawtooth and Pioneer Mountains in south-central Idaho, where a cluster of stations are reporting 65-75% of median SWE, and southwest New Mexico with three stations below 20% of median SWE. Basins in southeastern Arizona are at 80% of median SWE.
In Alaska, well below normal snowpack continues for the Kenai Peninsula. Several SNOTEL stations are reporting less than 50% of median SWE; Grouse Creek Divide SNOTEL at 700 ft. elevation is reporting the lowest at 30% of median SWE. The persistence of low snowpack for the region into mid-winter is most recently being driven by a lack of precipitation (meteorological drought) and large storms in January, but large SWE deficits initially developed in December with heavy precipitation, warm temperatures, and rain at lower elevations (warm snow drought).
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, 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 precipitation and snow water equivalent (SWE) relative to climatology at Paradise, Washington. In the past 30 days SWE has increased 29.4 inches and precipitation has totaled 29.6 inches. The SWE percentile has made a remarkable improvement from 8th on January 1, 2020 to 62nd on February 3, 2020. Graphics can be found at NRCS.
USDA Natural Resources Conservation Service (NRCS) SNOTEL stations in the Washington Cascades on February 3, 2020. Nearly all stations have improved to near-to-above normal precipitation shown by white or green filled circles. However, several stations that are mostly below 1200 m are still reporting SWE less than 80% of median.